[Early Draft] Splicing prototype

[optout21]

Splicing prototype, draft; PR for visibility (not intended for merging as for now).

Current baseline: v0.0.123, plus a not-latest version of dual-funding PR #2302 .

For start, run the splicing integration tests:

RUSTFLAGS="--cfg=splicing" cargo test -p lightning splic

See tests in functional_tests_splicing.rs.

Some documentation is available in README.md.