Rename Sodium::Pwhash#key_derive to #derive_key.
Rename Sodium::Pwhash#kdf_derive to #derive_kdf.
Rename Sodium::CryptoBox#encrypt_easy to #encrypt.
Rename Sodium::CryptoBox#decrypt_easy to #decrypt.
Rename Sodium::SecretBox#encrypt_easy to #encrypt.
Rename Sodium::SecretBox#decrypt_easy to #decrypt.
Remove redundant Sodium::SecretBox#encrypt methods.