Add bitcoin support

[LesterEvSe]

Adds experimental Bitcoin support: SimplicityHL programs can now be compiled against Bitcoin jets. A program picks its chain with a header at the top of the entry file:

target bitcoin;

Without it nothing changes and Elements jets are used, same as target elements;. The header is behind -Z bitcoin, and the Bitcoin backend behind the unstable-bitcoin Cargo feature.

In current state of PR rust-simplicity is patched to 49e0891, the head of the stacked PRs 385–388, and the patch can be replaced with a version bump once those are released.

To test we can use:

cargo test --features unstable-bitcoin
BITCOIND_EXE=/path/to/bitcoind just check_bitcoin_integration

The second one spends examples/bitcoin_p2pk.simf on a local regtest node.
I ran it against Bitcoin Inquisition v29.2-inq-simplicity.

Checklist

• If this PR adds or changes a language feature, it is used in
  examples/kitchen_sink.simf, and test-data/kitchen_sink.json is
  regenerated.