hash: expose hash context functions to Rust

hash: expose hash context functions to Rust

We'd like to be able to hash our data in Rust using the same contexts as
in C.  However, we need our helper functions to not be inline so they
can be linked into the binary appropriately.  In addition, to avoid
managing memory manually and since we don't know the size of the hash
context structure, we want to have simple alloc and free functions we
can use to make sure a context can be easily dynamically created.

Expose the helper functions and create alloc, free, and init functions
we can call.

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