k256: impl ff and group traits

[tarcieri]

• Impls ff::{Field, PrimeField} on Scalar
• Impls group::{Group, Curve} on ProjectivePoint

[tarcieri]

Lots of clippy complaints about unnecessary borrows now. I'm wondering if we need to add #[inline] on the various owned arithmetic operations to remove those without impacting performance. Edit: applied clippy fixes and posted benchmarks below.

It also seems like there are a lot of inherent methods on Scalar and ProjectivePoint which also have similar or identical methods on the trait. Perhaps we should get rid of the duplicated inherent methods and just use the trait?

[tarcieri]

Applied clippy's suggested fixes with the new arithmetic impls on owned types. I do really like how it improves the code quality (by removing "eye of sauron"-style borrowed arithmetic).

Ran the existing benchmarks... it looked like mostly improvements except for normalize:

field element operations/normalize_weak
                        time:   [1.6940 ns 1.7020 ns 1.7113 ns]
                        change: [+4.7960% +5.6804% +6.5209%] (p = 0.00 < 0.05)
                        Performance has regressed.
Found 7 outliers among 100 measurements (7.00%)
  1 (1.00%) low mild
  5 (5.00%) high mild
  1 (1.00%) high severe
field element operations/normalize
                        time:   [1.7010 ns 1.7075 ns 1.7142 ns]
                        change: [+1.4923% +3.0551% +4.6100%] (p = 0.00 < 0.05)
                        Performance has regressed.

cc @fjarri

[fjarri]

I wouldn't worry about it. The change is small, and when we're talking about an operation that takes literally 1ns, I doubt that these measurements are very accurate. On my laptop it takes ~4ns, and I don't see any change between this PR and master.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 24.50980% with 77 lines in your changes missing coverage. Please review.

Project coverage is 59.43%. Comparing base (d81c5c0) to head (4fe63b0).
Report is 893 commits behind head on master.

Files with missing lines	Patch %	Lines
k256/src/arithmetic/scalar.rs	12.76%	41 Missing ⚠️
k256/src/arithmetic/projective.rs	18.75%	26 Missing ⚠️
k256/src/arithmetic/scalar/scalar_4x64.rs	0.00%	4 Missing ⚠️
k256/src/arithmetic/scalar/scalar_8x32.rs	0.00%	4 Missing ⚠️
k256/src/arithmetic/affine.rs	0.00%	2 Missing ⚠️

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@@            Coverage Diff             @@
##           master     #164      +/-   ##
==========================================
- Coverage   60.64%   59.43%   -1.22%     
==========================================
  Files          25       25              
  Lines        3601     3688      +87     
==========================================
+ Hits         2184     2192       +8     
- Misses       1417     1496      +79     

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