Add a modexp example guest program + benchmark

[moodlezoup]

Add a guest program performing modexp (a la EVM) operations to examples/ and a corresponding benchmark to 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).

[0xAndoroid]

Closing this as slate