Gen bindings with rustc --pretty=expanded instead of reading files

Gen bindings with rustc --pretty=expanded instead of reading files

Instead of walking individual rust files and reading the AST from
those, we instead call
`RUSTC_BOOTSTRAP=1 cargo rustc --profile=check -- -Zunstable-options --pretty=expanded`
and let it create one giant lib.rs which we can parse as a whole.
This allows us to parse a post-macro crate, working with structs
and functions created inside macros just fine. It does require
handling a few things that we didn't previously, most notably Clone
via `impl ::core::clone::Clone` blocks instead of just looking for
`#![derive(Clone)]`.

This ends up resolving a few types slightly differently, resulting
in different bindings, but only in ways which don't impact the
runtime.