Add Math.pow functionality

I found that I needed the Math.pow functionality for something, but realised there was no `Math.[fn](a,b)` function as an exported function like `Math.[fn](a)`. So I thought I may as well add it.

A few things I wasn't sure about:
* I wasn't entirely sure what to call the new math export function, so happy to change it.
* Do I need to add more tests elsewhere? Have I done the tests in the right place?
* Have I missed anything?