ecdsa: use newly refactored sec1::EncodedPoint

[tarcieri]

Uses the new sec1::EncodedPoint type introduced in RustCrypto/traits#264

[codecov-commenter]

Codecov Report

Merging #131 into master will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #131   +/-   ##
=======================================
  Coverage   43.37%   43.37%           
=======================================
  Files           6        6           
  Lines         219      219           
=======================================
  Hits           95       95           
  Misses        124      124           

Impacted Files	Coverage Δ
ecdsa/src/lib.rs	47.61% <ø> (ø)
ecdsa/src/verifier.rs	0.00% <0.00%> (ø)

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