ed25519: add PKCS#8 support

[tarcieri]

Adds an ed25519::pkcs8::KeypairBytes type which impls the FromPrivateKey/ToPrivateKey traits from the pkcs8 crate.

[codecov-commenter]

Codecov Report

Merging #381 (0d9d396) into master (5286a97) will increase coverage by 5.90%.
The diff coverage is 91.37%.

@@            Coverage Diff             @@
##           master     #381      +/-   ##
==========================================
+ Coverage   53.82%   59.72%   +5.90%     
==========================================
  Files           9       11       +2     
  Lines         314      370      +56     
==========================================
+ Hits          169      221      +52     
- Misses        145      149       +4     

Impacted Files	Coverage Δ
ed25519/tests/serde.rs	100.00% <ø> (ø)
ed25519/src/lib.rs	60.00% <50.00%> (ø)
ed25519/src/pkcs8.rs	88.88% <88.88%> (ø)
ed25519/tests/pkcs8.rs	100.00% <100.00%> (ø)

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