ecdsa: rename Curve::ElementSize => FieldSize

[tarcieri]

See: RustCrypto/traits#282

[codecov-commenter]

Codecov Report

Merging #137 into master will not change coverage.
The diff coverage is 75.00%.

@@           Coverage Diff           @@
##           master     #137   +/-   ##
=======================================
  Coverage   30.03%   30.03%           
=======================================
  Files           8        8           
  Lines         313      313           
=======================================
  Hits           94       94           
  Misses        219      219           

Impacted Files	Coverage Δ
ecdsa/src/dev/curve.rs	0.00% <ø> (ø)
ecdsa/src/hazmat.rs	0.00% <ø> (ø)
ecdsa/src/signer.rs	0.00% <ø> (ø)
ecdsa/src/signer/rfc6979.rs	0.00% <ø> (ø)
ecdsa/src/verifier.rs	0.00% <ø> (ø)
ecdsa/src/lib.rs	47.61% <60.00%> (ø)
ecdsa/src/asn1.rs	70.47% <100.00%> (ø)

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