Get parser-c working on stable Rust

[tcr]

Some examples of why this is not yet the case are use of the box syntax and procedural macros (see #6).

[tcr]

FnBox will be used after #22 lands, so rust-lang/rust#28796 now applies here. Might be able to use https://stbuehler.github.io/rustdocs/boxfnonce/boxfnonce/struct.BoxFnOnce.html though.

[tcr]

With #27 I think there's no blockers for stable Rust :) until we add impl Trait support and a tokio interface

[birkenfeld]

Heh :) There is a fair bit of box patterns (and there will be more in analysis), without which AST node matches are somewhat more annoying to handle.