Tapscript Multisig Output Script Descriptors.

Defines multi_a() and sortedmulti_a(), descriptors for multisig spending rules inside Taproot scripts.

Like the older multi() and sortedmulti(), these take a threshold and a list of keys, so that a set number of them must sign. The difference is that they produce the kind of script used inside Taproot, work only inside tr(), and allow up to 999 keys.