flua: Add hash module

Add lua bindings to hashing functions. sha256 is available. sha256.new
craetes a new object. sha256.update updates the digest. sha256.digest
returns the digest as a binary string and resets the
context. sha256.hexdigest returns the digest as a string of hex digits
and then resets the cotnext.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D43872