ed25519: updates for ring-compat v0.2.1

[tarcieri]

VerifyKey was renamed to VerifyingKey

[codecov-commenter]

Codecov Report

Merging #291 (eb9ce18) into master (a9ee79b) will not change coverage.
The diff coverage is n/a.

❗ Current head eb9ce18 differs from pull request most recent head 4c8e719. Consider uploading reports for the commit 4c8e719 to get more accurate results

@@           Coverage Diff           @@
##           master     #291   +/-   ##
=======================================
  Coverage   55.29%   55.29%           
=======================================
  Files           9        9           
  Lines         293      293           
=======================================
  Hits          162      162           
  Misses        131      131           

Impacted Files	Coverage Δ
ed25519/src/lib.rs	65.30% <ø> (ø)

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