Refactor ProjectiveArithmetic trait

[tarcieri]

Accompanying changes for RustCrypto/traits#300.

[codecov-commenter]

Codecov Report

Merging #179 into master will increase coverage by 0.14%.
The diff coverage is 80.30%.

@@            Coverage Diff             @@
##           master     #179      +/-   ##
==========================================
+ Coverage   58.39%   58.53%   +0.14%     
==========================================
  Files          25       25              
  Lines        3778     3765      -13     
==========================================
- Hits         2206     2204       -2     
+ Misses       1572     1561      -11     

Impacted Files	Coverage Δ
k256/src/arithmetic.rs	100.00% <ø> (ø)
k256/src/arithmetic/field/field_10x26.rs	0.00% <ø> (ø)
k256/src/arithmetic/field/field_5x52.rs	0.00% <ø> (ø)
k256/src/arithmetic/scalar/scalar_4x64.rs	0.00% <ø> (ø)
k256/src/arithmetic/scalar/scalar_8x32.rs	91.70% <ø> (ø)
k256/src/ecdsa/sign.rs	31.48% <0.00%> (+0.57%)	⬆️
p256/src/arithmetic.rs	100.00% <ø> (ø)
k256/src/arithmetic/scalar.rs	78.62% <50.00%> (+1.84%)	⬆️
k256/src/ecdsa/recoverable.rs	58.66% <50.00%> (-3.68%)	⬇️
p256/src/arithmetic/scalar.rs	78.77% <76.19%> (+1.36%)	⬆️
... and 8 more

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