k256/p256: wire up ecdsa::Signer

[tarcieri]

Adds a type alias for ecdsa::signer::Signer, which provides a high-level ECDSA signing interface.

[codecov-commenter]

Codecov Report

Merging #101 into master will increase coverage by 0.06%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
+ Coverage   52.66%   52.72%   +0.06%     
==========================================
  Files          19       19              
  Lines        3437     3433       -4     
==========================================
  Hits         1810     1810              
+ Misses       1627     1623       -4     

Impacted Files	Coverage Δ
k256/src/arithmetic/scalar.rs	73.48% <ø> (+0.67%)	⬆️
k256/src/arithmetic/scalar/scalar_8x32.rs	0.00% <ø> (ø)
k256/src/ecdsa.rs	0.00% <0.00%> (ø)
k256/src/ecdsa/recoverable.rs	0.00% <ø> (ø)
p256/src/arithmetic/scalar.rs	63.39% <ø> (+0.41%)	⬆️
p256/src/arithmetic/scalar/blinding.rs	0.00% <0.00%> (ø)
p256/src/ecdsa.rs	0.00% <0.00%> (ø)

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