... the latter for being used in OP_CHECKMULTISIG. Now each of
CHECKDATASIG/CHECKSIG/CHECKMULTISIG calls its own custom encoding checker.
Rationale: We are switching to a different strategy where 64-byte ECDSA
signatures will now also be banned from OP_CHECKMULTISIG once
SCRIPT_ENABLE_SCHNORR activates. (schnorr spec has not yet been updated
on this point). In a child Diff, that behaviour will be added.