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/api/ui.c

@@ -111,7 +111,7 @@ void remote_ui_disconnect(uint64_t channel_id)
   }
   UIData *data = ui->data;
   kv_destroy(data->call_buf);
-  pmap_del(uint64_t)(&connected_uis, channel_id);
+  pmap_del(uint64_t)(&connected_uis, channel_id, NULL);
   ui_detach_impl(ui, channel_id);
 
   // Destroy `ui`.