rust: add a small wrapper around the hashfile code

rust: add a small wrapper around the hashfile code

Our new binary object map code avoids needing to be intimately involved
with file handling by simply writing data to an object implement Write.
This makes it very easy to test by writing to a Cursor wrapping a Vec
for tests, and thus decouples it from intimate knowledge about how we
handle files.

However, we will actually want to write our data to an actual file,
since that's the most practical way to persist data.  Implement a
wrapper around the hashfile code that implements the Write trait so that
we can write our object map into a file.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>