Ope in rust

This is an Order-preserving encryption (OPE) lib inspired by cryptdb's ope implementation.

It is a pure rust implementation, no c dependencies needed.

It is also written for no-std targets (thanks to num-traits) and works in wasm.

The max value to encrypt is 65532

```rust use opers::getope;

fn main() { let k = b"this is a key 10".to_owned();

let ope = get_ope(&k);

let a = ope.encrypt(21).unwrap();
let b = ope.encrypt(65531).unwrap();
let c = ope.encrypt(65532).unwrap();

assert!(a < b);
assert!(b < c);

} ```