Prepare proc_macro for decoupling it from the rest of the compiler.
This is #49219 up to the point where the bridge is introduced. Aside from moving some code around, the largest change is the rewrite of `proc_macro::quote` to be simpler and do less introspection.
I'd like to also extend `quote!` with `${stmt;...;expr}` instead of just `$variable` (and maybe even `$(... $iter ...)*`), which seems pretty straight-forward now, but I don't know if/when I should.
r? @alexcrichton or @dtolnay cc @jseyfried @petrochenkov
bd455ef165