interactive-tx: Add dual-funding flow, using the interactive tx protocol

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

.aspell.en.pws

@@ -23,6 +23,7 @@ nhops
 retransmitted
 dev
 tradeoff
+kiloweight
 mixHeader
 uint
 hopsData
@@ -115,6 +116,31 @@ delayedsig
 hopDataSize
 I'th
 segwit
+SegWit
+sizeof
+varint
+tlv
+rbf
+RBF
+accepter
+accepter's
+subtype
+subsubtype
+redeemScript
+UTXO
+utxo
+utxos
+scriptPubkey
+scriptPubKey
+scriptPubKeys
+RBFs
+ack'd
+scriptlen
+btc
+sats
+prevoutnum
+prevtxid
+ints
 htlc
 htlcs
 ChaCha
@@ -386,3 +412,8 @@ csv
 CHECKSIGVERIFY
 IFDUP
 sats
+workflow
+PUSHDATA
+perkw
+prev
+vout

01-messaging.md

@@ -316,11 +316,11 @@ The 2-byte `len` field indicates the number of bytes in the immediately followin
 
 The channel is referred to by `channel_id`, unless `channel_id` is 0 (i.e. all bytes are 0), in which case it refers to all channels.
 
-The funding node:
+The funding node using channel establishment `open_channel`:
   - for all error messages sent before (and including) the `funding_created` message:
     - MUST use `temporary_channel_id` in lieu of `channel_id`.
 
-The fundee node:
+The fundee node using channel establishment v1 (`accept_channel`):
   - for all error messages sent before (and not including) the `funding_signed` message:
     - MUST use `temporary_channel_id` in lieu of `channel_id`.