rust: add functionality to hash an object

In a future commit, we'll want to hash some data when dealing with an
object map.  Let's make this easy by creating a structure to hash
objects and calling into the C functions as necessary to perform the
hashing.  For now, we only implement safe hashing, but in the future we
could add unsafe hashing if we want.  Implement Clone and Drop to
appropriately manage our memory.  Additionally implement Write to make
it easy to use with other formats that implement this trait.

While we're at it, add some tests for the various hashing cases.

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

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