use proc_macro instead of proc_macro2

[Freax13]

This pr tries to make the proc_macro crate accessible inside webassembly by implementing proc_macro's unstable interal trait proc_macro::bridge::Server and forwarding all the calls over ffi.

This pr is still work in progress. Todo:

• [X] get the demo working
• [x] implement all remaining functions
• [x] add documentation
• [x] feature gate all the unstable code and readd the stable proc_macro2 code
• [x] support jit?

closes #42

[Freax13]

@dtolnay I'd appreciate a review