Implement O(1) OP_IF/NOTIF/ELSE/ENDIF logic
Implement O(1) OP_IF/NOTIF/ELSE/ENDIF logic

This optimization was first suggested by Sergio Demian Lerner in
https://bitslog.wordpress.com/2017/04/17/new-quadratic-delays-in-bitcoin-scripts/.
The implementation follows the suggested approach there, but with a slightly
simpler representation.

peercoin/peercoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,peercoin/peercoin,peercoin/peercoin