Attempt to decrypt a private-box message, using your secret key.
If you were an intended recipient then the decrypted message is
returned as Some(Vec<u8>). If it was not for you, then None
will be returned.
Takes the message you want to encrypt, and an array of recipient public keys.
Returns a message that is encrypted to all recipients and openable by them
with private_box::decrypt. The number of recipients must be between 1 and 32.