Implement a "phase 2" API for sign_closing_transaction

[devrandom]

This allows the signer to figure out which output is ours without potentially complicated parsing.

[codecov]

Codecov Report

Merging #1064 (eebc0a9) into main (4c4d99b) will increase coverage by 0.25%.
The diff coverage is 79.61%.

@@            Coverage Diff             @@
##             main    #1064      +/-   ##
==========================================
+ Coverage   90.87%   91.13%   +0.25%     
==========================================
  Files          65       65              
  Lines       33229    34569    +1340     
==========================================
+ Hits        30196    31503    +1307     
- Misses       3033     3066      +33     

Impacted Files	Coverage Δ
lightning/src/ln/chan_utils.rs	95.88% <78.08%> (-1.53%)	⬇️
lightning/src/ln/channel.rs	91.54% <80.76%> (+2.79%)	⬆️
lightning/src/chain/keysinterface.rs	94.86% <100.00%> (+0.17%)	⬆️
lightning/src/util/enforcing_trait_impls.rs	89.38% <100.00%> (+0.09%)	⬆️
lightning/src/ln/functional_tests.rs	97.47% <0.00%> (-0.04%)	⬇️

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[TheBlueMatt]

Generally LGTM.

[TheBlueMatt]

(here and in channel.rs) Calling this the "shutdown" script is a misnomer - the payout script is different from the shutdown one.

[devrandom]

Done

[TheBlueMatt]

I must've been high, I'm not sure what this means. Either script or shutdown_script are fine...

[valentinewallace]

Just nits, LGTM

[TheBlueMatt]

Only diff since Val's ack are addressing comments, will land after CI.