Implement LN-symmetry

[bigspider]

It would be great to have an implementation of LN-symmetry, both to develop useful CCV use cases for lightning, but also to experiment with composition of lightning and other constructions (like pools, Ark, etc.).

We should probably assume that the various useful opcodes are available (OP_CAT, OP_CHECKSIGFROMSTACK, OP_CHECKTEMPLATEVERIFY). All of the above are available in the container.

Useful notes:

• HTLC output aggregation - notes from @halseth on how to use CCV to improve HTLC handling.
• This gist shows that OP_CCV can roughly simulate OP_CSFS, although in a somewhat awkward way; therefore, we should probably just have OP_CSFS.