The old signature was `secret key -> public key -> T.t -> nonce:nonce -> ciphertext`. The present signature has 2 issues, to my mind:

1. One cannot curry a secret key to box for different public keys (perhaps this is a feature so you don't put sensitive material into closures?).
2. You can curry the nonce which will always be a security error.

What was difficult to use about the old signature?