add support for power operator

[Philipp15b]

Caesar should have support for a power operator. Z3 even supports powers on Real and Int types natively! The exponent is always a non-negative integer if I understood correctly.

Since we often use powers in expectations, our implementation should support EUReal bases as well. Extra cases: \infty^0 == 0 and \infty^exponent == \infty for `exponent != 0.