Add Invert to MontyForm supertraits

[andrewwhitehead]

No description provided.

[tarcieri]

Note that this is technically a breaking change, which we really need to stop making, though this one is unlikely to have user-facing impact. It would involve yanking the existing stable releases again though like last time, which is somewhat disruptive.

Perhaps we should also seal Integer which would make future changes like this non-breaking.

[andrewwhitehead]

Hm, I figured since it was additive it wouldn't be disruptive. Another potential one is adding from_montgomery since you can export the integer in Montgomery form but not import it.

[andrewwhitehead]

Also doesn't have to be done now, implementers can use U: UnsignedWithMontyForm<MontyForm: Invert<Output=CtOption<U::MontyForm>>> or something like that.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.99%. Comparing base (2484f32) to head (508a278).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1226      +/-   ##
==========================================
+ Coverage   87.97%   87.99%   +0.01%     
==========================================
  Files         184      184              
  Lines       21238    21244       +6     
==========================================
+ Hits        18684    18693       +9     
+ Misses       2554     2551       -3     

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[tarcieri]

Hm, I figured since it was additive it wouldn't be disruptive

Right now it's possible for downstream crates to impl the Integer trait for their own types. If they do that, and we add a new trait requirement they don't impl, it will break their code.

I doubt anyone is doing that yet, so this is a largely hypothetical scenario, but it's one we should respect in our SemVer contract.

We can avoid this situation entirely by sealing the Integer trait so downstream crates can't impl it.

[tarcieri]

I opened a PR to seal the Integer trait: #1227