refactor(map): avoid duplicated khash_t types for values

This reduces the total number of khash_t instantiations from 22 to 8.

Make the khash internal functions take the size of values as a runtime
parameter. This is abstracted with typesafe Map containers which
are still specialized for both key, value type.

Introduce `Set(key)` type for when there is no value.

Refactor shada.c to use Map/Set instead of khash directly.
This requires `map_ref` operation to be more flexible.
Return pointers to both key and value, plus an indicator for new_item.
As a bonus, `map_key` is now redundant.

Instead of Map(cstr_t, FileMarks), use a pointer map as the FileMarks struct is
humongous.

Make `event_strings` actually work like an intern pool instead of wtf it
was doing before.

src/nvim/memory.c

@@ -664,6 +664,7 @@ char *arena_memdupz(Arena *arena, const char *buf, size_t size)
 # include "nvim/getchar.h"
 # include "nvim/grid.h"
 # include "nvim/mark.h"
+# include "nvim/msgpack_rpc/channel.h"
 # include "nvim/ops.h"
 # include "nvim/option.h"
 # include "nvim/os/os.h"
@@ -823,6 +824,7 @@ void free_all_mem(void)
 
   ui_free_all_mem();
   nlua_free_all_mem();
+  rpc_free_all_mem();
 
   // should be last, in case earlier free functions deallocates arenas
   arena_free_reuse_blks();