Add restore (#11)

[kibernetikos]

Overview:

This Pull Request addresses Issue #11 . The issue highlights the absence of a functionality to save and restore Keypair instances, which is a critical feature for many applications, particularly those requiring key persistence across sessions or environments.

Changes:

1. Implementation of Keypair::new Method:

• Following the community member's suggestion, i've implemented the restore method within the Keypair struct. This method allows the creation of a Keypair object from separate byte vectors for the public and private keys.
• The method is straightforward to use: it takes two Vec arguments, one for the public key and one for the private key, and constructs a Keypair object.

2. Improving Usability and Flexibility:

• The addition of the restore method significantly increases the library's usability, particularly in scenarios where keys need to be stored and later reconstructed, such as in distributed systems, backups, or when transferring keys between different parts of a system.