Draw contents from a byte string instead of generating arbitrary hashes

This generates more realistic contents and avoids spurious HashCollisions when generating a set of objects using these hypothesis strategies, at the cost of slightly worse "boundary checking" (i.e. we won't check contents with a length > 4096 bytes).

Depends on !86 (closed).

Test Plan

tox tests still pass

---

Migrated from D2739 (view on Phabricator)