Further fixups for dalek-cryptography#108

The build is still broken when rand is disabled, we are building with

```
..., default-features = false, features = ["alloc", "nightly", "serde", "u64_backend"] }
```

src/secret.rs

@@ -19,6 +19,7 @@ use curve25519_dalek::digest::Digest;
 use curve25519_dalek::edwards::CompressedEdwardsY;
 use curve25519_dalek::scalar::Scalar;
 
+#[cfg(feature = "rand")]
 use rand::{CryptoRng, RngCore};
 
 use sha2::Sha512;