[xml] Initial implementation of core:encoding/xml.

A from-scratch XML implementation, loosely modeled on the [spec](https://www.w3.org/TR/2006/REC-xml11-20060816).

Features:
		- Supports enough of the XML 1.0/1.1 spec to handle the 99.9% of XML documents in common current usage.
		- Simple to understand and use. Small.

Caveats:
		- We do NOT support HTML in this package, as that may or may not be valid XML.
		  If it works, great. If it doesn't, that's not considered a bug.

		- We do NOT support UTF-16. If you have a UTF-16 XML file, please convert it to UTF-8 first. Also, our condolences.
		- <[!ELEMENT and <[!ATTLIST are not supported, and will be either ignored or return an error depending on the parser options.

TODO:
- Optional CDATA unboxing.
- Optional `&gt;`, `&#32;`, `&#x20;` and other escape substitution in tag bodies.
- Test suite

MAYBE:
- XML writer?
- Serialize/deserialize Odin types?

core/encoding/xml/example/xml_example.odin

@@ -0,0 +1,55 @@
+package xml_example
+
+import "core:encoding/xml"
+import "core:mem"
+import "core:fmt"
+
+Error_Handler :: proc(pos: xml.Pos, fmt: string, args: ..any) {
+
+}
+
+FILENAME :: "../../../../tests/core/assets/xml/nl_NL-xliff-1.0.xliff"
+DOC      :: #load(FILENAME)
+
+OPTIONS  :: xml.Options{
+	flags            = {
+		.Ignore_Unsupported, .Intern_Comments,
+	},
+	expected_doctype = "",
+}
+
+_main :: proc() {
+	using fmt
+
+	println("--- DOCUMENT TO PARSE  ---")
+	println(string(DOC))
+	println("--- /DOCUMENT TO PARSE ---\n")
+
+	doc, err := xml.parse(DOC, OPTIONS, FILENAME, Error_Handler)
+	defer xml.destroy(doc)
+
+	xml.print(doc)
+
+	if err != .None {
+		printf("Parse error: %v\n", err)
+	} else {
+		println("DONE!")
+	}
+}
+
+main :: proc() {
+	using fmt
+
+	track: mem.Tracking_Allocator
+	mem.tracking_allocator_init(&track, context.allocator)
+	context.allocator = mem.tracking_allocator(&track)
+
+	_main()
+
+	if len(track.allocation_map) > 0 {
+		println()
+		for _, v in track.allocation_map {
+			printf("%v Leaked %v bytes.\n", v.location, v.size)
+		}
+	}	
+}