Use a hash-and-diff strategy to produce a diff layout for block-based documents

Summary:
Depends on D20835. Ref T13425. Ref T13414. When a document has a list of content blocks, we may not be able to diff it directly, but we can hash each block and then diff the hashes (internally "diff" also does approximately the same thing).

We could do this ourselves with slightly fewer layers of indirection, but: diff already exists; we already use it; we already have a bunch of abstractions on top of it; and it's likely much faster on large inputs than the best we can do in PHP.

Test Plan:

Maniphest Tasks: T13425, T13414

Differential Revision: https://secure.phabricator.com/D20836