hash: add a function to look up hash algo structs

hash: add a function to look up hash algo structs

In C, it's easy for us to look up a hash algorithm structure by its
offset by simply indexing the hash_algos array.  However, in Rust, we
sometimes need a pointer to pass to a C function, but we have our own
hash algorithm abstraction.

To get one from the other, let's provide a simple function that looks up
the C structure from the offset and expose it in Rust.

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