Add p:file-hash

[JDziurlaj]

I need a way to verify the hash of certain documents entering an XProc pipeline. I cannot use p:hash as I need to hash the actual bytes that may vary by encoding (UTF-8, UTF-16, EBCDIC, etc.)

The proposed step would behave like p:load, but in addition, one of the output ports would contain a hashed value of the source. Options would be mainly in line with p:hash.

There might be some desire for hashing to apply to href using URI schemes other than file, making this perhaps a more general facility.