Add low_r signature grinding

[devrandom]

default on, can be turned off via a feature gate

Fixes #1387

[codecov-commenter]

Codecov Report

Merging #1388 (8d7b38f) into main (216225c) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1388      +/-   ##
==========================================
+ Coverage   90.82%   90.84%   +0.02%     
==========================================
  Files          73       73              
  Lines       41245    42497    +1252     
  Branches    41245    42497    +1252     
==========================================
+ Hits        37460    38606    +1146     
- Misses       3785     3891     +106     

Impacted Files	Coverage Δ
lightning/src/ln/channel.rs	88.78% <ø> (-0.43%)	⬇️
lightning/src/chain/keysinterface.rs	93.58% <100.00%> (ø)
lightning/src/ln/chan_utils.rs	95.51% <100.00%> (ø)
lightning/src/ln/channelmanager.rs	87.27% <100.00%> (+2.41%)	⬆️
lightning/src/util/crypto.rs	100.00% <100.00%> (ø)
lightning/src/util/events.rs	30.75% <0.00%> (-2.47%)	⬇️
lightning/src/ln/functional_tests.rs	97.45% <0.00%> (+0.35%)	⬆️

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[arik-so]

awesome, thanks!

[valentinewallace]

It looks like this PR introduced a buncha warnings in CI, FWIW

[devrandom]

@valentinewallace I don't quite see how these changes would result in those warnings. I assume you mean in channel.rs?

[valentinewallace]

@devrandom Yeah, I'm not sure how either, I just see that the warnings appear on this PR's merge. I see them in "Test on Rust stable with net-tokio" but they may be elsewhere too

[devrandom]

Oh, I know, that's in the mod tests. One of the tests is only compiled in when grinding is off, because it hardcodes the check of a bunch of signatures.

[devrandom]

hmm... that test is a bit of a pain. perhaps we can have it skip the signatures when comparing?

[devrandom]

@TheBlueMatt since you wrote test_commitment! - how do you feel about skipping the comparison of the signature?