2.22.1: Don't crash on large hunks

Before this release, if a hunk was too large riff would take forever and
use up unreasonable amounts of memory, sometimes to the point of
crashing.

With this release, if a hunk is too large riff won't refine it but just
pass it through as-is, working around the related performance issues.

Issue: https://github.com/walles/riff/issues/35