Convert `op_id` and `party_id` to enums

[nph4rd]

Description

Currently, we're working with op_id and party_id as a usize variables, and then we use these variables to "match" values, for example, like the following:

https://github.com/OpenMined/sycret/blob/fc776e1a9c517aebdd7a705d448eb1865cda6479/src/lib.rs#L14

I think we could maybe use an enum instead. For example, in the case of op_id:

enum OpId {
    Eq,
    Le,
}

This would be more idiomatic for Rust and could help us prevent errors my limiting options to the enum variants.

Are you interested in working on this improvement yourself?

Yes.