Use PrimeCurve trait

[tarcieri]

Switches from weierstrass::Curve to the PrimeCurve trait added in RustCrypto/traits#737.

[codecov-commenter]

Codecov Report

Merging #413 (0db42f0) into master (75bfa19) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #413   +/-   ##
=======================================
  Coverage   62.55%   62.55%           
=======================================
  Files          28       28           
  Lines        3974     3974           
=======================================
  Hits         2486     2486           
  Misses       1488     1488           

Impacted Files	Coverage Δ
k256/src/arithmetic/affine.rs	78.70% <ø> (ø)
k256/src/ecdsa/recoverable.rs	59.72% <ø> (ø)
p256/src/arithmetic/affine.rs	86.66% <ø> (ø)

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