Renames Update to SignedBatch.
Checks Batch equality via a hash of the InInstructions. That prevents needing
to keep the Batch in node state or TX introspect.
Provides a DST, and associated metadata as beneficial.
Also utilizes MuSig's context to session-bind. Since set_keys_messages also
binds to set, this is semi-redundant, yet that's appreciated.
It originally wasn't an enum so software which had yet to update before an
integration wouldn't error (as now enums are strictly typed). The strict typing
is preferable though.
