3727: Introduce ra_proc_macro r=matklad a=edwin0cheng
This PR implemented:
1. Reading dylib path of proc-macro crate from cargo check , similar to how `OUTDIR` is obtained.
2. Added a new crate `ra_proc_macro` and implement the foot-work for reading result from external proc-macro expander.
3. Added a struct `ProcMacroClient` , which will be responsible to the client side communication to the External process.
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>
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