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/ex_cmds_defs.h

@@ -233,5 +233,5 @@ typedef struct {
 typedef struct {
   char *sub;            ///< Previous replacement string.
   Timestamp timestamp;  ///< Time when it was last set.
-  list_T *additional_elements;  ///< Additional data left from ShaDa file.
+  AdditionalData *additional_data;  ///< Additional data left from ShaDa file.
 } SubReplacementString;