k256/p256: use shared util functions

[tarcieri]

Migrated to the elliptic-curve crate in RustCrypto/traits#281

[codecov-commenter]

Codecov Report

Merging #149 into master will increase coverage by 0.06%.
The diff coverage is 97.97%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
+ Coverage   50.89%   50.95%   +0.06%     
==========================================
  Files          21       20       -1     
  Lines        3482     3458      -24     
==========================================
- Hits         1772     1762      -10     
+ Misses       1710     1696      -14     

Impacted Files	Coverage Δ
k256/src/arithmetic.rs	81.27% <ø> (ø)
k256/src/arithmetic/field.rs	91.21% <ø> (ø)
k256/src/arithmetic/field/field_montgomery.rs	0.00% <0.00%> (ø)
k256/src/arithmetic/scalar.rs	73.68% <ø> (ø)
k256/src/arithmetic/scalar/scalar_4x64.rs	76.19% <ø> (ø)
k256/src/arithmetic/scalar/scalar_8x32.rs	0.00% <ø> (ø)
p256/src/arithmetic.rs	81.75% <ø> (ø)
k256/src/arithmetic/dev.rs	100.00% <100.00%> (ø)
p256/src/arithmetic/field.rs	77.20% <100.00%> (ø)
p256/src/arithmetic/scalar.rs	63.57% <100.00%> (ø)
... and 1 more

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