MAST with OP_EVAL and OP_CAT

Inspired by Pieter's Tree Signatures, I believe Merkleized Abstract Syntax Trees (MAST) could be implemented with only OP_CAT and OP_EVAL (BIP12).

The idea is very simple. Using a similar example in Pieter's paper,

scriptSig = <sig> <serialized sub-script 10> Z1 0 1 1 X6 1 K9 0 <serialized script> scriptPubKey = DUP HASH160 <hash serialized script> EQUALVERIFY EVAL serialized script = 8 PICK SHA256 (SWAP IF SWAP ENDIF CAT SHA256)*4 <R> EQUALVERIFY EVAL

This will run the 10-th sub-script, when there are 11 sub-scripts in the MAST

I think this is the easiest way to enable MAST since the reference implementation for BIP12 is already there. We could enable OP_CAT only inside OP_EVAL so this will be a pure softfork.

Ref: Tree Signatures: https://blockstream.com/2015/08/24/treesignatures/ BIP12: https://github.com/bitcoin/bips/blob/master/bip-0012.mediawiki