Of course, I'm happy to try writing this.

But there is one thing that I don't know what the best solution is: how to
reuse the code
in the function `adjust_frame_size' (defined in src/frame.c, used by
`set-frame-size')?
I want to add two arguments: `new_x' and `new_y'. When  `new_x' and `new_y'
are the
same with the current frame position, execute the original branch
(`set_window_size_hook'
or `resize_frame_window'); otherwise, call a move and resize function.
But I have to rename this function to `adjust_frame_position_and_size'. For
compatibility,
I will also create a wrapper function called `adjust_frame_size', in which
`adjust_frame_position_and_size' is called with the current frame position
as the last two args
so that the frame will be only resized but not moved.
Do you think this is an appropriate solution?