#153 Optimize hash use during re-signing

[Murray S. Kucherawy]

CPU cycles could be saved during re-signing with header binding if a hash state could be copied from a header canonicalization handle prior to the final signature being added to it so that a different signature could be added to the same header set without re-running all the headers.

When re-signing a signing handle, this is possibly fairly easy. When re-signing a verifying handle, the header hash is generated start-to-finish in one API call, so there would have to be a hook in there to make the copy/copies, or a list of re-signing handles to which the state needs to be copied.