Mandatory activation of taproot deployment.

Proposed a way to switch on Taproot that would, if needed, have forced miners to signal support before a deadline.

Under this plan, Taproot would switch on once 90% of blocks in a period signalled support. If that hadn't happened near the end, blocks that failed to signal could be rejected, so miners couldn't hold back a widely backed change forever. It was built to fit alongside 'Speedy Trial', a separate method released in Bitcoin Core.