Use RFC 8032 functions added to x/crypto/ed25519

[tmthrgd]

This was added in golang/crypto@5ba7f63082 and can replace the workaround from #458 / #591.

[codecov-io]

Codecov Report

Merging #715 into master will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #715      +/-   ##
==========================================
+ Coverage   55.76%   55.77%   +0.01%     
==========================================
  Files          39       39              
  Lines       10753    10751       -2     
==========================================
  Hits         5996     5996              
+ Misses       3673     3672       -1     
+ Partials     1084     1083       -1

Impacted Files	Coverage Δ
dnssec_privkey.go	96.77% <100%> (ø)	⬆️
dnssec_keyscan.go	70.89% <50%> (+0.74%)	⬆️

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