We refactor `Commitments.kt` to better match `eclair` and clean up
pattern matching (use exhaustive, future-proof pattern matching).

We also remove alternative feerate sigs, which aren't necessary
anymore since we can now use package relay. The ACINQ node will
stop sending `update_fee` and will keep the commitment feerate
at `1 sat/byte`.

Error handling was missing from a few places (incorrectly assuming that
nonces were already provided and signature couldn't fail). We also use
exhaustive pattern matching everywhere, and clean-up the nonces and
funding tx index parameters.

We also revert some formatting nits, which make the file inconsistent.

The nonce management was incomplete for mutual close RBF (see changes
to `Negotiating.kt`). We must add unit tests to catch those bugs.

Mutual close was also not working properly because `revoke_and_ack`
nonces weren't taken into account in the `ShuttingDown` state.

We also refactor the nonce fields in the various channel states, in
which some nonces where unnecessary (e.g. already contained in the
`shutdown` message).

We make the ordering of fields consistent: `remoteNextCommitNonces`
always comes right after `commitments`, since it should always be
filled (for taproot channels) and is necessary to update the
`commitments`.