github.com/agl/ed25519 is deprecated and contains known bugs

The module github.com/agl/ed25519 is no longer maintained due to having been merged into go-crypto (and since go-1.13, subsequently into the standard library) and by its creator's own admission contains at least one known bug they can't be bothered to fix there. It really would make sense to switch to the maintained version... Unfortunately this is not as simple as changing the import because to the best of my knowledge, the submodule "extra25519" - which is what obfs4proxy actually uses - has not been merged in.