RA Proc Macro Server
This library is able to call compiled Rust custom derive dynamic libraries on arbitrary code.
The general idea here is based on https://github.com/fedochet/rust-proc-macro-expander.
But we adapt it to better fit RA needs:
* We use `tt` for proc-macro `TokenStream` server, it is easier to manipulate and interact with
RA than `proc-macro2` token stream.
* By **copying** the whole rustc `lib_proc_macro` code, we are able to build this with `stable`
rustc rather than `unstable`. (Although in general ABI compatibility is still an issue)…