This commit adds the interactive transaction construction protcol, as
well as the first practical example of using it, v2 of channel
establishment.

Note that for v2 we also update the channel_id, which now uses the hash
of the revocation_basepoints. We move away from using the funding
transaction id, as the introduction of RBF* makes it such that a single
channel may have many funding transaction id's over the course of
its lifetime.

*Later, also splicing

So that it knows what open_channel2 is responding to.

Spotted-By: @rustyrussell

…x protocol

Suggested-By: Antoine Riard @ariard

Remove the fee_step in favor of using an explicit bump feerate, with
the constraint that it must be at least 1/64th greater than the last
sent rate.

Suggested-By: @rustyrussell

As @t-bast points out, we define 'point' in 01, which covers the
encoding.

Suggested-By: @t-bast

Spotted-By: @t-bast

Suggested-By: @t-bast

"Failing the channel" may not be the right action in all cases where the
interactive construction protocol is used (e.g. closes), so we move
reference to it as a mitigation technique specifically to the usage of
the interactive protocol in the v2 open sequence.

Suggested-By: @t-bast

Suggested-By: @t-bast

For open_channel2 + accept_channel2 we use the opener's basepoint and a
zeroed out point for the accepter.  This is because we don't know what
the accepter's basepoint is yet. (It's transmitted in accept_channel2)

Pointed-Out-By: @t-bast

Suggested-By: @t-bast

Noticed-By: @SomberNight

Reported-by: @SomberNight

Suggested-by: @instagibbs

P2SH-wrapped inputs are segwit inputs and should be eligible for
spending, however the spec as written didn't permit them

Reported-by: @SomberNight

P2SH outputs have larger dust requirements than native SegWit (and
Taproot) scripts; we disallow the creation of them on funding txs as a
they're problematic for relay when allowing dust limits below 546 sats
(see lightning#894)

Suggested-by: @t-bast

Requested-by: @t-bast

input_count of 1 times min witness weight of 110, is 110. not 107

the sum of 612 is correct, however.

Reported-By: @SimonVrouwe

* Move RBF inside the interactive-tx protocol
* Use same tlvs as open_channel / accept_channel
* Require native segwit
* Fix a few typos and whitespaces issues (thanks markdown linter)

Dual funding proposed codec updates

And add `channel_type` tlv.

We use 1/24 instead of 1/64 to produce larger steps in fee-bumping.

Add signature data, remove wrapped segwit inputs, update calculations
to be more explicit about how to figure fee

If both peers contribute an equal amount to a interactive tx,
peer with "lower" pubkey sends first

The weight of a witness stack for a p2wpkh low-r signature is 107 bytes.

Don't show up in the CLN generator w/o this