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/highlight_group.c

@@ -1420,7 +1420,7 @@ void do_highlight(const char *line, const bool forceit, const bool init)
 void free_highlight(void)
 {
   ga_clear(&highlight_ga);
-  map_destroy(cstr_t, int)(&highlight_unames);
+  map_destroy(cstr_t, &highlight_unames);
   arena_mem_free(arena_finish(&highlight_arena));
 }