Ah, yes, there are no signatures in x/crypto/nacl. There's https://github.com/agl/ed25519, however it will be slower, as there's no assembly.

Edit: oops, looks like signatures were added 2 months ago (https://github.com/golang/crypto/blob/master/ed25519/). (I guess it's the same agl's code). There is a detached version (Sign/Verify).