k256/p256: use new elliptic_curve::encoding module

[tarcieri]

Impls the new FromBytes traits introduced in RustCrypto/traits#247, and changes to_bytes() to return ElementBytes.

[codecov-commenter]

Codecov Report

Merging #117 into master will increase coverage by 0.07%.
The diff coverage is 67.07%.

@@            Coverage Diff             @@
##           master     #117      +/-   ##
==========================================
+ Coverage   51.53%   51.60%   +0.07%     
==========================================
  Files          20       20              
  Lines        3489     3484       -5     
==========================================
  Hits         1798     1798              
+ Misses       1691     1686       -5     

Impacted Files	Coverage Δ
k256/src/arithmetic/field/field_10x26.rs	0.00% <0.00%> (ø)
k256/src/arithmetic/field/field_montgomery.rs	0.00% <0.00%> (ø)
k256/src/arithmetic/scalar/scalar_8x32.rs	0.00% <0.00%> (ø)
k256/src/ecdsa/recoverable.rs	0.00% <ø> (ø)
k256/src/ecdsa/signer.rs	0.00% <0.00%> (ø)
k256/src/ecdsa/verifier.rs	0.00% <0.00%> (ø)
p256/src/ecdsa.rs	0.00% <0.00%> (ø)
k256/src/arithmetic/scalar.rs	73.68% <54.54%> (+0.44%)	⬆️
p256/src/arithmetic/scalar.rs	64.23% <78.57%> (+0.42%)	⬆️
k256/src/arithmetic.rs	84.32% <100.00%> (-0.06%)	⬇️
... and 7 more

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