add POW(**) operator and tests

[cedric-h]

In a minute, I'll push an example that uses the POW operator to output a Sierpinski triangle.

[cedric-h]

Hmm, I'm going to have to be clever about how I write this intcode. I found a solution that doesn't require division or modulo, but it does use nested arrays. I should be able to fake it by having a reference array and another array that stores indexes into it, though.