Add the snapshot crate, which implements snapshotting at a low level

[gefjon]

Description of Changes

• Requires making BlobHash be Serialize and Deserialize. For arcane macro-ology reasons, this requires writing BlobHash::SIZE instead of Self::SIZE (it gets embedded in a visitor struct or something).
• Requires adding two new operators to BlobStore.
• Adds a return value to Page::save_content_hash, for convenience.
• Impls DerefMut for Pages.
• Scary change: adds Table::pages_mut. I think possibly this operator should be unsafe, since write access to the Pages allows an undisciplined caller to violate the Table's assumptions by corrupting a Page. It seems like an anti-pattern to mark a method unsafe on the grounds that misusing its return value can cause UB, but I don't see a plausible alternative without making most methods on Page unsafe. Open to feedback on this one!
• Adds Table::iter_pages_with_hashes, which iterates over all pages, ensuring their hashes are present before yielding.

API and ABI breaking changes

N/a

Expected complexity level and risk

3 - see above comment re: safety of Table::pages_mut.

Testing

Tested manually in #1279 . Not convinced there's a useful way write tests for this in isolation i.e. before integrating.

[gefjon]

As an alternative to pages_mut, I could expose a method Table::iter_hashed_pages&mut self) -> impl Iterator<Item = &Page> which updates the hashes internally before yielding, but doesn't give mutable access to the Page or the Pages. I like that better, and will update after lunch.