tor-key-forge: encapsulate `define_ed25519_keypair` macro dependencies

This re-exports the types/traits needed by the define_ed25519_keypair macro so that the macro caller doesn't need to import a bunch of extra packages in its Cargo.toml that it doesn't use, and so that the caller doesn't need a use prelude::* before invoking the macro. This makes the macro nicer to use for the caller, and should prevent the macro from causing "cannot find ... in this scope" errors.

@dgoulet Do you agree with this change?