refactor(shada): rework msgpack decoding without msgpack-c

This also makes shada reading slightly faster due to avoiding
some copying and allocation.

Use keysets to drive decoding of msgpack maps for shada entries.

src/nvim/context.h

@@ -1,6 +1,5 @@
 #pragma once
 
-#include <msgpack/sbuffer.h>
 #include <stddef.h>
 
 #include "klib/kvec.h"