R1:08a42254371f

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…