Use a consistent definition of Ed25519

aka "Just use Ristretto"

• Go:
  • Use github.com/oasisprotocol/curve25519-voi/primitives/x25519 for X25519 (#140).
  • Use github.com/oasisprotocol/oasis-core/go/common/crypto/signature for Ed25519.
  • 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:
  • 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.

Start with the checklist entries for client-sdk/go/go.mod, common::crypto::signature, and the Oasis runtime signature module, then review the existing Go and Rust changes referenced in the issue. The remaining work is to document why third parties should avoid crypto/ed25519 and to implement Ed25519 verification in TypeScript/JavaScript with the project's semantics; done means both unchecked items are addressed.