Add a SHA512 implementation

Add a SHA512 implementation

This largely follows the SHA256 style. I've named Rusty as the maintainer.

Currently the functions to add data of various sizes/endianness have not
been implemented: There are no public test vectors for these cases and
I believe most use cases are working on byte buffers. They can be added
later if desired.

The openssl implementation has been tested on x86-64, while the inbuilt
version has been tested on 32/64 bit, little/big endian boxes.

Signed-off-by: Jon Griffiths <jon_p_griffiths@yahoo.com>