BIP-119.2: Extensions for Lightning Symmetry

[jeremy]

This adds 3 new modes:

- 20 Byte Hash160 CTV

- 21 and 33 Byte CTV where last byte indicates second-to-top stack element to be included in hash

The extension data is beneficial to Lightning Symmetry applications as they need data inscribed in a block so that clients may delete old channel states.

One question to particularly address is if Hash160 is both preimage and collision resistant in this context, or if 20 bytes will be a security issue for lightning developers (I'm aware they use Hash160 for other applications)

see draft BIP here: https://github.com/bitcoin/bips/pull/1587