memory: Add free wrapper and refactor project to use it

We already use wrappers for allocation, the new `xfree` function is the
equivalent for deallocation and provides a way to fully replace the malloc
implementation used by Neovim.

src/nvim/api/vim.c

@@ -85,7 +85,7 @@ void vim_feedkeys(String keys, String mode, Boolean escape_csi)
       insert ? 0 : typebuf.tb_len, !typed, false);
 
   if (escape_csi) {
-      free(keys_esc);
+      xfree(keys_esc);
   }
 
   if (vgetc_busy)