sha2 - SHA-256/-384/-512 hashing

`local sha2 = require'sha2'`

A ffi binding of Aaron Gifford's SHA-2 implementation.

`sha2.sha256(s[, size]) -> s` `sha2.sha256(cdata, size) -> s` `sha2.sha384(s[, size]) -> s` `sha2.sha384(cdata, size) -> s` `sha2.sha512(s[, size]) -> s` `sha2.sha512(cdata, size) -> s`	Compute the SHA-2 hash of a string or a cdata buffer. Return the binary representation of the hash. To get the hex representation, use glue.tohex.
`sha2.sha256_digest() -> digest` `sha2.sha384_digest() -> digest` `sha2.sha512_digest() -> digest` `digest(s[, size])` `digest(cdata, size)` `digest() -> s`	Get a SHA-2 digest function that can consume multiple data chunks until called with no arguments when it returns the final SHA hash.