k256/p256: impl Zeroize for AffinePoint and FieldElement types

[tarcieri]

This is used by the generic ecdh implementation to zero out the transient shared secret value after it's been serialized to bytes.

[codecov-commenter]

Codecov Report

Merging #124 into master will decrease coverage by 0.29%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
- Coverage   51.63%   51.34%   -0.30%     
==========================================
  Files          21       21              
  Lines        3484     3504      +20     
==========================================
  Hits         1799     1799              
- Misses       1685     1705      +20     

Impacted Files	Coverage Δ
k256/src/arithmetic.rs	83.44% <0.00%> (-0.88%)	⬇️
k256/src/arithmetic/field.rs	91.21% <0.00%> (-0.90%)	⬇️
k256/src/arithmetic/field/field_10x26.rs	0.00% <0.00%> (ø)
k256/src/arithmetic/field/field_5x52.rs	92.22% <0.00%> (-0.66%)	⬇️
k256/src/arithmetic/field/field_impl.rs	89.70% <0.00%> (-5.61%)	⬇️
k256/src/arithmetic/field/field_montgomery.rs	0.00% <0.00%> (ø)
p256/src/arithmetic.rs	83.21% <0.00%> (-0.87%)	⬇️
p256/src/arithmetic/field.rs	77.60% <0.00%> (-0.63%)	⬇️
p256/src/ecdsa.rs	0.00% <ø> (ø)

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