Small API update

[LinuxHeki]

Hello, I updated the API a little bit:

1. Keypair.public field is now private and the public key can be accessed using Keypair.public() function. Now the user can't accidentally modify public field.
2. You can save keys to a file and next time read it and use it using Keypair::from_bytes() function.
3. More secure rng can be used using Keypair::random(rng) function.

[LinuxHeki]

I forgot to check if there are other pull requests... Damn it