Missing CHECKMULTISIGFROMSTACK

[maaku]

Now that Schnorr multi-signature aggregation has been removed, there is a need for a MULTISIG version of CHECKSIGFROMSTACK. This was not included in Alpha because Schnorr signatures allowed any multi-sig to reduce to a regular CHECKSIG.

[instagibbs]

MuSig deployment in future I think deprecates any real need for this. Closing for now.