refactor(api): use a unpacker based on libmpack instead of msgpack-c

Currently this is more or less a straight off reimplementation,
but this allow further optimizations down the line, especially
for avoiding memory allocations of rpc objects.

Current score for "make functionaltest; make oldtest" on a -DEXITFREE build:

is 117 055 352 xfree(ptr != NULL) calls (that's NUMBERWANG!).

src/nvim/api/vim.c

@@ -45,6 +45,7 @@
 #include "nvim/move.h"
 #include "nvim/msgpack_rpc/channel.h"
 #include "nvim/msgpack_rpc/helpers.h"
+#include "nvim/msgpack_rpc/unpacker.h"
 #include "nvim/ops.h"
 #include "nvim/option.h"
 #include "nvim/os/input.h"
@@ -2188,6 +2189,12 @@ void nvim__screenshot(String path)
   ui_call_screenshot(path);
 }
 
+Object nvim__unpack(String str, Error *err)
+  FUNC_API_FAST
+{
+  return unpack(str.data, str.size, err);
+}
+
 /// Deletes an uppercase/file named mark. See |mark-motions|.
 ///
 /// @note fails with error if a lowercase or buffer local named mark is used.