Provide "bin/files integrity" for debugging, maintaining and backfilling integrity hashes

Summary:
Ref T12470. Provides an "integrity" utility which runs in these modes:

• Verify: check that hashes match.
• Compute: backfill missing hashes.
• Strip: remove hashes. Useful for upgrading across a hash change.
• Corrupt: intentionally corrupt hashes. Useful for debugging.
• Overwrite: force hash recomputation.

Users normally shouldn't need to run any of this stuff, but this provides a reasonable toolkit for managing integrity hashes.

I'll recommend existing installs use bin/files integrity --compute all in the upgrade guidance to backfill hashes for existing files.

Test Plan:

• Ran the script in many modes against various files, saw expected operation, including:
• Verified a file, corrupted it, saw it fail.
• Verified a file, stripped it, saw it have no hash.
• Stripped a file, computed it, got a clean verify.
• Stripped a file, overwrote it, got a clean verify.
• Corrupted a file, overwrote it, got a clean verify.
• Overwrote a file, overwrote again, got a no-op.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12470

Differential Revision: https://secure.phabricator.com/D17629