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

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