PR for BIP 117: Tail call semantics

Bitcoin Core PR #12132 was opened on 2018-01-09.

Implements BIP 117 as policy rules for transaction relay, causing transactions which violate the new tail-call semantics to be treated as non-standard. This is not the soft-fork activation logic needed to use tail-call scripts in production.

Tail-call semantics is a sort of user-programmable P2SH which combines well with BIP 116’s MERKLEBRANCHVERIFY to allows for a script to commit to a practically unbounded number of code pathways, and then reveal the actual code pathway used at spend time, achieving a form of generalized MAST.

As of the time of this writing these PRs are still open and have not yet been merged.