Allow modern hunks to be stored deflated

Summary: Ref T4045. Ref T5179. When saving a modern hunk, deflate it if we have the function and deflating it will save a nontrivial number of bytes.

Test Plan:

• Used bin/hunks migrate to move some hunks over, saw ~70-80% compression on most standard hunks.
• Viewed changesets using compressed hunks.
• Profiled gzinflate() and verified the cost is trivial (<< 1ms) at least for normal diffs.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4045, T5179

Differential Revision: https://secure.phabricator.com/D9292