go: Use a faster ed25519 implementation

[Yawning]

This should force everything to use our ed25519 and x25519.

[codecov]

Codecov Report

Merging #1533 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1533      +/-   ##
==========================================
+ Coverage   52.94%   52.95%   +0.01%     
==========================================
  Files         272      272              
  Lines       27185    27185              
==========================================
+ Hits        14393    14397       +4     
+ Misses      11314    11311       -3     
+ Partials     1478     1477       -1

Impacted Files	Coverage Δ
go/common/crypto/mrae/api/api.go	22.91% <ø> (ø)	⬆️
...n/crypto/signature/signers/memory/memory_signer.go	55.26% <ø> (ø)	⬆️
go/common/crypto/signature/signature.go	37.07% <ø> (ø)	⬆️
...ommon/crypto/signature/signers/file/file_signer.go	54.21% <ø> (ø)	⬆️
go/scheduler/tendermint/tendermint.go	66.32% <0%> (+4.08%)	⬆️

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[Yawning]

Bring this up to date again.

While this branch has been sitting waiting for the courage to merge it, Go 1.13 has promoted the slow x/crypto/ed25519 code to crypto/ed25519. While this isn't a problem for us, if our dependencies switch to using the standard library import path, it will limit our ability to override the implementation.

Because this only really matters for tendermint, we can always just define our own Ed25519 implementation that tendermint will use, especially if we want to switch to ph or ctx there as well (and we should).

Edit: Can't define custom key types in tendermint, because crypto/encoding/amino is poorly implemented.

[Yawning]

For what it's worth, I'm of the opinion that this is good to go, the moment we get the ed25519 code cleared.

[peterjgilbert]

🎉