Expose conversions to and from integer representations to allow usage…

[gkgoat1]

… in downstream crates and network protocols.

[TitouanReal]

Thank you for contributing! May I ask in what situation you are using this crate?

I looked at your PR and i have some questions about the changes in shared_secret.rs. In what situation would you want to create a new shared secret from a Uint?

[gkgoat1]

Thank you for contributing! May I ask in what situation you are using this crate?

I am planning to use this crate as an implemention of CSIDH, specifically to replace x-wing as a post-quantum encryption system (x-wing has very large public keys, and for embedding in DNS and email, that may be an issue).

I looked at your PR and i have some questions about the changes in shared_secret.rs. In what situation would you want to create a new shared secret from a Uint?

I mostly added that method for completeness, since I could easily just hash the secret. Being able to serialize and deserialize types here would, however, be an important feature.