Work with Stable Rust

[ndmitchell]

It would be easier if Pyrefly used Stable Rust. Less rust_toolchain files, works better with distributions etc. I've eliminated a number of features, but the remaining ones which aren't stabilised in the next Rust version are:

• [ ] box_patterns
• [ ] const_type_name

Neither are likely to be stabilised anytime soon, so we need to work towards eliminating them.