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/msgpack_rpc/channel.c

@@ -478,9 +478,9 @@ static void on_request_event(Event event)
                                             result,
                                             &out_buffer));
   // All arguments were freed already, but we still need to free the array
-  free(args.items);
+  xfree(args.items);
   decref(channel);
-  free(e);
+  xfree(e);
 }
 
 static bool channel_write(Channel *channel, WBuffer *buffer)
@@ -601,7 +601,7 @@ static void unsubscribe(Channel *channel, char *event)
 
   // Since the string is no longer used by other channels, release it's memory
   pmap_del(cstr_t)(event_strings, event_string);
-  free(event_string);
+  xfree(event_string);
 }
 
 /// Close the channel streams/job and free the channel resources.
@@ -655,13 +655,13 @@ static void free_channel(Channel *channel)
   pmap_free(cstr_t)(channel->subscribed_events);
   kv_destroy(channel->call_stack);
   kv_destroy(channel->delayed_notifications);
-  free(channel);
+  xfree(channel);
 }
 
 static void close_cb(uv_handle_t *handle)
 {
-  free(handle->data);
-  free(handle);
+  xfree(handle->data);
+  xfree(handle);
 }
 
 static Channel *register_channel(void)
@@ -738,7 +738,7 @@ static WBuffer *serialize_request(uint64_t channel_id,
   WBuffer *rv = wstream_new_buffer(xmemdup(sbuffer->data, sbuffer->size),
                                    sbuffer->size,
                                    refcount,
-                                   free);
+                                   xfree);
   msgpack_sbuffer_clear(sbuffer);
   api_free_array(args);
   return rv;
@@ -757,7 +757,7 @@ static WBuffer *serialize_response(uint64_t channel_id,
   WBuffer *rv = wstream_new_buffer(xmemdup(sbuffer->data, sbuffer->size),
                                    sbuffer->size,
                                    1,  // responses only go though 1 channel
-                                   free);
+                                   xfree);
   msgpack_sbuffer_clear(sbuffer);
   api_free_object(arg);
   return rv;