Allow signing and verifying by hash #29
                
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I have a need to sign and verify messages that are not contiguous in memory. These added methods allow that, with a user provided closure providing the hash of the message.
The use case is that I am serializing a message on-the-fly from parts to sign or verify, without copying it into a contiguous buffer. I have implemented
DynDigestso that the user provided functions can more easily use Salty's sha512.