Add dynlib.initialize_symbols (#3071)

```
package example

import "core:dynlib"
import "core:fmt"

Symbols :: struct {
	// `foo_` is prefixed, so we look for the symbol `foo_add`.
	add: proc "c" (int, int) -> int,
	// We use the tag here to override the symbol to look for, namely `bar_sub`.
	sub: proc "c" (int, int) -> int `dynlib:"bar_sub"`,

	// Exported global (if exporting an i32, the type must be ^i32 because the symbol is a pointer to the export.)
	// If it's not a pointer or procedure type, we'll skip the struct field.
	hellope: ^i32,

	// Handle to free library.
	// We can have more than one of these so we can match symbols for more than one DLL with one struct.
	_my_lib_handle: dynlib.Library,
}

main :: proc() {
	sym: Symbols

	// Load symbols from `lib.dll` into Symbols struct.
	// Each struct field is prefixed with `foo_` before lookup in the DLL's symbol table.
	// The library's Handle (to unload) will be stored in `sym._my_lib_handle`. This way you can load multiple DLLs in one struct.
	count := dynlib.initialize_symbols(&sym, "lib.dll", "foo_", "_my_lib_handle")
	defer dynlib.unload_library(sym._my_lib_handle)
	fmt.printf("%v symbols loaded from lib.dll (%p).\n", count, sym._my_lib_handle)

	if count > 0 {
		fmt.println("42 + 42 =", sym.add(42, 42))
		fmt.println("84 - 13 =", sym.sub(84, 13))
		fmt.println("hellope =", sym.hellope^)
	}
}
```

core/dynlib/doc.odin

@@ -1,7 +1,11 @@
+//+build ignore
 /*
 Package core:dynlib implements loading of shared libraries/DLLs and their symbols.
 
 The behaviour of dynamically loaded libraries is specific to the target platform of the program.
 For in depth detail on the underlying behaviour please refer to your target platform's documentation.
+
+See `example` directory for an example library exporting 3 symbols and a host program loading them automatically
+by defining a symbol table struct.
 */
-package dynlib
+package dynlib

core/dynlib/example/example.odin

@@ -0,0 +1,36 @@
+package example
+
+import "core:dynlib"
+import "core:fmt"
+
+Symbols :: struct {
+	// `foo_` is prefixed, so we look for the symbol `foo_add`.
+	add: proc "c" (int, int) -> int,
+	// We use the tag here to override the symbol to look for, namely `bar_sub`.
+	sub: proc "c" (int, int) -> int `dynlib:"bar_sub"`,
+
+	// Exported global (if exporting an i32, the type must be ^i32 because the symbol is a pointer to the export.)
+	// If it's not a pointer or procedure type, we'll skip the struct field.
+	hellope: ^i32,
+
+	// Handle to free library.
+	// We can have more than one of these so we can match symbols for more than one DLL with one struct.
+	_my_lib_handle: dynlib.Library,
+}
+
+main :: proc() {
+	sym: Symbols
+
+	// Load symbols from `lib.dll` into Symbols struct.
+	// Each struct field is prefixed with `foo_` before lookup in the DLL's symbol table.
+	// The library's Handle (to unload) will be stored in `sym._my_lib_handle`. This way you can load multiple DLLs in one struct.
+	count := dynlib.initialize_symbols(&sym, "lib.dll", "foo_", "_my_lib_handle")
+	defer dynlib.unload_library(sym._my_lib_handle)
+	fmt.printf("%v symbols loaded from lib.dll (%p).\n", count, sym._my_lib_handle)
+
+	if count > 0 {
+		fmt.println("42 + 42 =", sym.add(42, 42))
+		fmt.println("84 - 13 =", sym.sub(84, 13))
+		fmt.println("hellope =", sym.hellope^)
+	}
+}

core/dynlib/example/lib.odin

@@ -0,0 +1,14 @@
+package library
+
+@(export)
+foo_add :: proc "c" (a, b: int) -> (res: int) {
+	return a + b
+}
+
+@(export)
+bar_sub :: proc "c" (a, b: int) -> (res: int) {
+	return a - b
+}
+
+@(export)
+foo_hellope: i32 = 42

core/dynlib/lib.odin

@@ -1,5 +1,12 @@
 package dynlib
 
+import "core:intrinsics"
+import "core:reflect"
+import "core:runtime"
+_ :: intrinsics
+_ :: reflect
+_ :: runtime
+
 /*
 A handle to a dynamically loaded library.
 */
@@ -12,11 +19,11 @@ library available to resolve references in subsequently loaded libraries.
 The paramater `global_symbols` is only used for the platforms `linux`, `darwin`, `freebsd` and `openbsd`.
 On `windows` this paramater is ignored.
 
-The underlying behaviour is platform specific.  
-On `linux`, `darwin`, `freebsd` and `openbsd` refer to `dlopen`.  
+The underlying behaviour is platform specific.
+On `linux`, `darwin`, `freebsd` and `openbsd` refer to `dlopen`.
 On `windows` refer to `LoadLibraryW`.
 
-**Implicit Allocators**  
+**Implicit Allocators**
 `context.temp_allocator`
 
 Example:
@@ -39,8 +46,8 @@ load_library :: proc(path: string, global_symbols := false) -> (library: Library
 /*
 Unloads a dynamic library.
 
-The underlying behaviour is platform specific.  
-On `linux`, `darwin`, `freebsd` and `openbsd` refer to `dlclose`.  
+The underlying behaviour is platform specific.
+On `linux`, `darwin`, `freebsd` and `openbsd` refer to `dlclose`.
 On `windows` refer to `FreeLibrary`.
 
 Example:
@@ -67,11 +74,11 @@ unload_library :: proc(library: Library) -> (did_unload: bool) {
 /*
 Loads the address of a procedure/variable from a dynamic library.
 
-The underlying behaviour is platform specific.  
-On `linux`, `darwin`, `freebsd` and `openbsd` refer to `dlsym`.  
+The underlying behaviour is platform specific.
+On `linux`, `darwin`, `freebsd` and `openbsd` refer to `dlsym`.
 On `windows` refer to `GetProcAddress`.
 
-**Implicit Allocators**  
+**Implicit Allocators**
 `context.temp_allocator`
 
 Example:
@@ -92,3 +99,72 @@ Example:
 symbol_address :: proc(library: Library, symbol: string) -> (ptr: rawptr, found: bool) #optional_ok {
 	return _symbol_address(library, symbol)
 }
+
+/*
+Scans a dynamic library for symbols matching a struct's members, assigning found procedure pointers to the corresponding entry.
+Optionally takes a symbol prefix added to the struct's member name to construct the symbol looked up in the library.
+Optionally also takes the struct member to assign the library handle to, `__handle` by default.
+
+This allows using one struct to hold library handles and symbol pointers for more than 1 dynamic library.
+
+Returns:
+* -1 if the library could not be loaded.
+* The number of symbols assigned on success.
+
+See doc.odin for an example.
+*/
+initialize_symbols :: proc(symbol_table: ^$T, library_name: string, symbol_prefix := "", handle_field_name := "__handle") -> (count: int) where intrinsics.type_is_struct(T) {
+	assert(symbol_table != nil)
+	ok:     bool
+	handle: Library
+
+	if handle, ok = load_library(library_name); !ok {
+		return -1
+	}
+
+	// `symbol_table` must be a struct because of the where clause, so this can't fail.
+	ti := runtime.type_info_base(type_info_of(T))
+	s, _ := ti.variant.(runtime.Type_Info_Struct)
+
+	// Buffer to concatenate the prefix + symbol name.
+	prefixed_symbol_buf: [2048]u8 = ---
+
+	sym_ptr: rawptr
+	for field_name, i in s.names {
+		// Calculate address of struct member
+		field_ptr := rawptr(uintptr(rawptr(symbol_table)) + uintptr(s.offsets[i]))
+
+		// If we've come across the struct member for the handle, store it and continue scanning for other symbols.
+		if field_name == handle_field_name {
+			(^Library)(field_ptr)^ = handle
+			continue
+		}
+
+		// We're not the library handle, so the field needs to be a pointer type, be it a procedure pointer or an exported global.
+		if !(reflect.is_procedure(s.types[i]) || reflect.is_pointer(s.types[i])) {
+			continue
+		}
+
+		// Let's look up or construct the symbol name to find in the library
+		prefixed_name: string
+
+		// Do we have a symbol override tag?
+		if override, tag_ok := reflect.struct_tag_lookup(reflect.Struct_Tag(s.tags[i]), "dynlib"); tag_ok {
+			prefixed_name = string(override)
+		}
+
+		// No valid symbol override tag found, fall back to `<symbol_prefix>name`.
+		if len(prefixed_name) == 0 {
+			offset := copy(prefixed_symbol_buf[:], symbol_prefix)
+			copy(prefixed_symbol_buf[offset:], field_name)
+			prefixed_name = string(prefixed_symbol_buf[:len(symbol_prefix) + len(field_name)])
+		}
+
+		// Assign procedure (or global) pointer if found.
+		if sym_ptr, ok = symbol_address(handle, prefixed_name); ok {
+			(^rawptr)(field_ptr)^ = sym_ptr
+			count += 1
+		}
+	}
+	return
+}