+ script layout and readability

fix and make scripts smaller

We'll just re-use the funding key instead. Alternatively, we could use
the _other_ party's key here.

Doesn't add anything atm, so might as well drop it.

This also adds the new partial signature encoding which uses the final
nonce which is needed for final signature combination.

…ady known.

…ion.

…signed`.

In this commit, we build off the prior JIT nonce proposal modifying the
following:

  * The funding flow returns to being symmetric. Both `open_channel` and
    `accept_channel` carry nonces.

  * The `shutdown` message once again transmit a nonce that'll be used
    by both sides to sign the shutdown transaction. A symmetric nonce
    approach is used here as both sides already know what the other will
    use for their nonce, and there's only a single message to sign.

  * The `closing_signed` message now only carries a 32-byte
    `partial_sig` value, as the nonces has already been transmitted
    ahead of time. Additionally, the existing "negotiation" feature has
    been done away with. Cooperative close will now always succeed after
    a single round, as the initiator is the only that ultimately pays
    fees.

bolt-simple-taproot: make funding symmetric, add new type for shutdown

- clarify `output_key_y_parity`
- add usage of htlc keys for anchors in case no balance output exists
- fix `2:` -> `2.` markdown ordered list
- clarify `SIGHASH_SINGLE|SIGHASH_ANYONECANPAY` usage in HTLC second level txs