Refactor magic numbers in token.rs

r? anyone

Add bindings for start and ends of keyword ranges; use bindings in match arms.

Also, fixed latent bug that inspired this change: the pattern in `is_any_keyword` had not been updated to match the new range of reserved keyword identifiers.

(I briefly tried to expose the latent bug, but `is_any_keyword` is currently only called in contexts where a failure of this kind merely causes a bit more fruitless compilation before `check_reserved_keywords` is called by the parser, which correctly tags `sizeof` as reserved.)