Add SHA512_Final_Len() et al

An optimisation: programmers can now easily take just a part
of the digest without needing an intermediate buffer to hold
the entire digest
Cosmetic improvements to "str.h"
Add strn_digest_passphrase()

Uses SHA512 internally
Accept '#passphrase' for bundle secret arguments

As well as a hex string.  Applies to CLI and also REST API.
Maximum pass phrase length is 80 ASCII chars.