Added example/test in symm.rs for encrypting a private key with a symmetric cipher

//!       \xFB\x3C\x5E\xC4\x59\x72\x4A\xF4\x7C\xA1",

//!     &ciphertext[..]);

//! ```

//!

//! Encrypting an assymetric key with a symmetric cipher

//!

//! ```

//! use openssl::rsa::{Padding, Rsa};

//! use openssl::symm::Cipher;

//!

//! // Generate keypair and encrypt private key:

//! let keypair = Rsa::generate(2048).unwrap();

//! let cipher = Cipher::aes_256_cbc();

//! let pubkey_pem = keypair.public_key_to_pem_pkcs1().unwrap();

//! let privkey_pem = keypair.private_key_to_pem_passphrase(cipher, b"Rust").unwrap();

//! // pubkey_pem and privkey_pem could be written to file here.

//!

//! // Load private and public key from string:

//! let pubkey = Rsa::public_key_from_pem_pkcs1(&pubkey_pem).unwrap();

//! let privkey = Rsa::private_key_from_pem_passphrase(&privkey_pem, b"Rust").unwrap();

//!

//! // Use the asymmetric keys to encrypt and decrypt a short message:

//! let msg = b"Foo bar";

//! let mut encrypted = vec![0; pubkey.size() as usize];

//! let mut decrypted = vec![0; privkey.size() as usize];

//! let len = pubkey.public_encrypt(msg, &mut encrypted, Padding::PKCS1).unwrap();

//! assert!(len > msg.len());

//! let len = privkey.private_decrypt(&encrypted, &mut decrypted, Padding::PKCS1).unwrap();

//! let output_string = String::from_utf8(decrypted[..len].to_vec()).unwrap();

//! assert_eq!("Foo bar", output_string);

//! println!("Decrypted: '{}'", output_string);

//! ```

use std::cmp;

use std::ptr;

use libc::c_int;