Refactor and fix FIXME's in mtwt hygiene code

- Moves mtwt hygiene code into its own file
- Fixes FIXME's which leads to ~2x speed gain in expansion pass
- It is now @-free
auto merge of #12711 : edwardw/rust/mtwt, r=pcwalton

- Moves mtwt hygiene code into its own file
- Fixes FIXME's which leads to ~2x speed gain in expansion pass
- It is now @-free