Rename ecdsa::{Signer, Verifier} => {SigningKey, VerifyKey}

[tarcieri]

Ala RustCrypto/signatures#140

[codecov-commenter]

Codecov Report

Merging #153 into master will decrease coverage by 0.06%.
The diff coverage is 6.66%.

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
- Coverage   60.28%   60.21%   -0.07%     
==========================================
  Files          21       21              
  Lines        3555     3559       +4     
==========================================
  Hits         2143     2143              
- Misses       1412     1416       +4     

Impacted Files	Coverage Δ
k256/src/ecdsa.rs	85.71% <ø> (ø)
k256/src/ecdsa/recoverable.rs	68.29% <ø> (ø)
k256/src/ecdsa/sign.rs	32.60% <0.00%> (ø)
k256/src/ecdsa/verify.rs	48.27% <0.00%> (ø)
p256/src/ecdsa.rs	83.33% <100.00%> (ø)

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