Use stdlib when possible

[fasmat]

This updates ed25519 to use the optimised stdlib implementation when it can and only use our own code for Sign2, Verify2 and ExtractPublicKey since those are not available in the standard library.

Benchmarks on Macbook M1:

BenchmarkPublicKeyExtraction-6              6666            176173 ns/op              32 B/op          1 allocs/op

BenchmarkSigningExt-6                      36211             33105 ns/op              64 B/op          1 allocs/op
BenchmarkSigning-6                         54192             22148 ns/op               0 B/op          0 allocs/op

BenchmarkVerificationExt-6                 13506             89068 ns/op               0 B/op          0 allocs/op
BenchmarkVerification-6                    25872             46401 ns/op               0 B/op          0 allocs/op

[fasmat]

These are the benchmarks on my machine for the Sign and Verify functions on the master branch:

BenchmarkSigning-6                         36337             32931 ns/op              64 B/op          1 allocs/op
BenchmarkVerification-6                    13368             90031 ns/op               0 B/op          0 allocs/op

[pigmej]

So from my side it makes sense. I cannot assess all the side effects, though. Checked both m1 and amd64.

Having separate opinion from @dshulyak is required.

[dshulyak]

i don't understand why spacemesh library needs to wrap any other lib
if i had to do it, i would create a lib named ed25519extract (or similar) and leave only sign/extract/verify functions that work with key extraction

but if there is some preference doing it this way i don't mind

[fasmat]

I see one reason to do this: If we change/remove the API of this package we break existing software using it.

This would be a "soft" migration away from our implementation of ed25519 where we don't break any existing software using this library, while at the same time allowing users of it a soft transistion to crypto/ed25519 similar how the transition from golang.org/x/net/context to context was handled. The former is now also just a wrapper around the later.