sealed boxes for Rust

This is a pure Rust implementation of libsodium sealed boxes.

Usage:

```rust // Recipient: create a new key pair let recipientkp = sealedbox::KeyPair::create();

// Sender: encrypt the message for the recipient whose public key is recipientkp.pk let msg = b"test"; let ciphertext = sealedbox::seal(msg, recipient_kp.pk);

// Recipient: decrypt the ciphertext using the key pair let decryptedmsg = sealedbox::open(&ciphertext, &recipient_kp).unwrap();

asserteq!(msg[..], decryptedmsg); ```