Rename longest_match_slow to longest_match_roll

Rename longest_match_slow to longest_match_roll

The "slow" variant of longest_match uses a 3-byte rolling hash to seed
its offset-search lookups after a match has been found. Rename the
template gate, the functable entry, and all arch-specific instantiations
from *_slow to *_roll to reflect what the variant actually uses, so a
separate integer-hash offset-search variant can coexist under its own
name.

Pure rename, no behavior change.