Draft: Remove all feature gates

[ambaxter]

I've created a branch that requires no feature gates and compiles for the most part on beta.

The only remaining nightly-only functionality is Span.join which is nightly only: https://doc.rust-lang.org/proc_macro/struct.Span.html#method.join . Open issue is rust-lang/rust#54725

All macro compiles panic on https://github.com/SergioBenitez/Pear/blob/master/codegen/src/parser.rs#L168

[macpp]

@ambaxter this is panicking because you are actually using Span::join() from proc_macro2 crate https://docs.rs/proc-macro2/1.0.18/proc_macro2/struct.Span.html#method.join (which will always return None on non-nightly rust). You should probably do something similar to pr #25 (or just unwrap_or(Span::call_site()) , but that's last resort)

[SergioBenitez]

Thank you! Migrating the crate to stable is/was unfortunately more involved than this PR would allude. The efforts to migrate the crate were also already underway. I've pushed the completed migration at 333c9bf. Pear now compiles on stable!