reorganize proc_macro crate

[cyrgani]

Revival of #134401, as a prerequisite for #130856.

This tries to implement the design outlined in #134401 (comment).

r? @tgross35

[bjorn3]

This tries to implement the design outlined in #134401 (comment).

An alternative way to implement #130856 I think would be to have just another server impl that uses the bridge similar to what rustc does. That seems to me like it would avoid unnecessary abstraction. Basically have a function that calls Client::expand1(some_proc_macro_func).run(&SameThread, NoRustcServer::new(), input, true) with NoRustcServer containing all code to run the proc macro outside rustc. Or maybe not exactly that to allow functions that are not exactly shaped like proc macros with a TokenStream input and output, but at least something that internally sets up all thread local storage to register NoRustcServer as the server to communicate with in the place of rustc.