mrustc now has (half of) a borrow checker

I've spent the last year and a half working on a borrow checker for mrustc, and have just merged the fruits of that work. I say it's half of a borrow checker, as this only includes the propagation of lifetime annotations through the program.

https://github.com/thepowersgang/mrustc/pull/314

For those who haven't heard of it, mrustc is my long-running project to create a compiler for rust that can be used to compile rustc without needing an existing rustc binary. It is able to bootstrap rustc 1.19, 1.29, 1.39 and 1.54 from source.