Add BytesMontgomery method to Scalar type

`Point` has `BytesMontgomery` which can be used to convert Ed25519 public key to X25519 public key quite easily.

However, there's no such thing for `Scalar`. Conversion should be quite simple. `libsodium` has [crypto_sign_ed25519_sk_to_curve25519](https://github.com/jedisct1/libsodium/blob/7d15af325d1bc6f20dd04df979c82c16e0e2ce2f/src/libsodium/crypto_sign/ed25519/ref10/keypair.c#L71) which does exactly this.