TaggedHash for BOLT 12 signing function

TaggedHash for BOLT 12 signing function

The function used to sign BOLT 12 messages only takes a message digest.
This doesn't allow signers to independently verify the message before
signing nor does it allow them to derive the necessary signing keys, if
needed.

Introduce a TaggedHash wrapper for a message digest, which each unsigned
BOLT 12 message type constructs upon initialization. Change the signing
function to take AsRef<TaggedHash>, which each unsigned type implements.
This allows the signing function to take any unsigned message and obtain
its tagged hash.