Move `from_binary` and `from_file` to `ObjectStore`

The shortcuts `MemStore.from_binary` and `MemStore.from_file` still work as documented but the methods need to be moved for upcoming synced file usage.
Improve counting capabilities.

- Add `count_*` variants (analogous to `Array#count`) that work like `find_*`.
- Add `length` as default, make `size` an alias.