Use a consistent definition of Ed25519

[Yawning]

aka "Just use Ristretto"

• Go:
  • [x] Use github.com/oasisprotocol/curve25519-voi/primitives/x25519 for X25519 (#140).
  • [x] Use github.com/oasisprotocol/oasis-core/go/common/crypto/signature for Ed25519.
  • [x] Fix the replace directives under client-sdk/go/go.mod, though nothing in this code-base should import the stdlib's X2551/Ed25519 code (#140).
  • [ ] Third parties that chose to use crypto/ed25519 instead of what we provide, are causing issues for themselves, so this should be documented.
• Rust:
  • [x] Don't import ed25519-dalek, use common::crypto::signature from github.com/oasisprotocol/oasis-core/runtime (Requires https://github.com/oasisprotocol/oasis-core/pull/3946) (#140).
• Typescript/JS:
  • [ ] Someone that can stomach writing TypeScript/JS needs to implement Ed25519 signature verification with our semantics.