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/lua/treesitter.c

@@ -231,9 +231,9 @@ int tslua_remove_lang(lua_State *L)
   const char *lang_name = luaL_checkstring(L, 1);
   bool present = pmap_has(cstr_t)(&langs, lang_name);
   if (present) {
-    char *key = (char *)pmap_key(cstr_t)(&langs, lang_name);
-    pmap_del(cstr_t)(&langs, lang_name);
-    xfree(key);
+    cstr_t key;
+    pmap_del(cstr_t)(&langs, lang_name, &key);
+    xfree((void *)key);
   }
   lua_pushboolean(L, present);
   return 1;