ecdsa: use PrimeCurve trait

[tarcieri]

Switches from weierstrass::Curve to the PrimeCurve trait added in RustCrypto/traits#737.

[codecov-commenter]

Codecov Report

Merging #357 (7e2a98e) into master (5961aaa) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #357   +/-   ##
=======================================
  Coverage   53.65%   53.65%           
=======================================
  Files           9        9           
  Lines         328      328           
=======================================
  Hits          176      176           
  Misses        152      152           

Impacted Files	Coverage Δ
ecdsa/src/der.rs	81.33% <ø> (ø)
ecdsa/src/hazmat.rs	0.00% <ø> (ø)
ecdsa/src/lib.rs	37.50% <ø> (ø)
ecdsa/src/rfc6979.rs	95.00% <ø> (ø)
ecdsa/src/sign.rs	0.00% <ø> (ø)
ecdsa/src/verify.rs	0.00% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5961aaa...7e2a98e. Read the comment docs.