Trim down and reduce compile times

[mystor]

Right now proc-macro2 stable makes up a decent amount of the build time for syn-master.

It shouldn't be too hard to strip down its dependencies and make the parsing code compile a bit faster, especially once #4 lands.

[alexcrichton]

Are you thinking of any particularly high priority piece? Wins in compile times nowadays are gained generally from "compile less code" but I'm not sure how much we can remove from this crate...

[mystor]

No, I don't think this is super high priority. I think I just remembered seeing the stable implementation doing some unnecessary work while parsing which we could potentially get away without. Nothing in specific.

[dtolnay]

I'll go ahead and close since this issue does not seem particularly actionable or useful to remain open. I would still welcome any PRs to help compile time in this or related crates.