Add a modexp example guest program + benchmark

Add a guest program performing modexp (a la EVM) operations to [examples/](https://github.com/a16z/jolt/tree/main/examples) and a corresponding benchmark to [e2e_profiling.rs](https://github.com/a16z/jolt/blob/main/jolt-core/benches/e2e_profiling.rs). The bitlength of the base, modulus, and exponent should be configurable, as well as the number of iterations (similar to the `sha2-chain` example).