ecdsa: rename ScalarBytes to ElementBytes

[tarcieri]

Per RustCrypto/traits#246

[codecov-commenter]

Codecov Report

Merging #112 into master will not change coverage.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##           master     #112   +/-   ##
=======================================
  Coverage   45.97%   45.97%           
=======================================
  Files           6        6           
  Lines         211      211           
=======================================
  Hits           97       97           
  Misses        114      114           

Impacted Files	Coverage Δ
ecdsa/src/hazmat.rs	0.00% <ø> (ø)
ecdsa/src/lib.rs	61.76% <80.00%> (ø)
ecdsa/src/asn1.rs	71.69% <100.00%> (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 4124ce3...6245087. Read the comment docs.