regex(3): Refactor fast/slow stepping bits in the matching engine

[kevans]

Summary

Adding features for matching is fairly straightforward, but requires some
duplication because of this fast/slow setup. They can be fairly trivially combined
into a single walk(), so do it to make it less error prone.

Test Plan

Run regex(3), sed, grep, gsed tests

[cem]

lib/libc/regex/engine.c

870

style nit: spaces around +

[kevans]

• Address style nit

[emaste]

lib/libc/regex/engine.c

482

walk inside of assert() is OK?

[kevans]

lib/libc/regex/engine.c

482

I believe in this case it should be fine; it's a sanity walk that shouldn't alter state since we should have exhausted the substring at that point.

[kevans]

Ping @emaste

[emaste]

Approved.