Taproot: SegWit version 1 spending rules.

Adds Taproot, a new kind of output that can be spent with one signature or with other conditions that stay hidden unless used.

Taproot coins are locked to a single key and, if wanted, to other spending conditions too. If everyone agrees, a spend looks like any ordinary one-signature payment and the extra conditions stay hidden. If not, only the condition actually used is revealed. This improves privacy and saves space.