Scoped userdata performance improvements

[benniekiss]

I recently ran into the performance impacts of using scoped userdata when writing some lua bindings for pest. My scoped approach was about 15x slower than simply converting all of the pest ast to an owned data structure. However, I lost a lot of the pest machinery, and I had to re-implement a lot of private functions to make it work as I want/need.

I started researching the prior discussions on scoped userdata performance and TypeId 'static requirements, and I ran across this crate, https://github.com/dtolnay/typeid. It provides TypeId functionality for non-static types, and I was wondering if this had been evaluated or considered for use in mlua. As the lack of TypeId support seems to be a significant cause of the performance impacts, I thought it may be worth an attempt at using the crate, but I am very new to rust, so I may be entirely missing the mark here :-P