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/main.c

@@ -6,7 +6,6 @@
 #endif
 #include <assert.h>
 #include <limits.h>
-#include <msgpack/pack.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>