k256: use NonZeroScalar for ECDSA signature components

[tarcieri]

Simplifies checks that r and s are non-zero by using the NonZeroScalar type.

[codecov-commenter]

Codecov Report

Merging #144 into master will increase coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   50.75%   50.81%   +0.05%     
==========================================
  Files          21       21              
  Lines        3491     3487       -4     
==========================================
  Hits         1772     1772              
+ Misses       1719     1715       -4     

Impacted Files	Coverage Δ
k256/src/ecdsa/recoverable.rs	0.00% <0.00%> (ø)
k256/src/ecdsa/verifier.rs	0.00% <0.00%> (ø)

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