sets of exponents

Since you can apply the `@fastpow` macro to a whole block of code, in principle it could recognize when you are computing multiple powers (e.g. `x^2`, `x^5`, and `x^7`) of a single variable, and then compute an optimal set of multiplications for the set, sharing operations wherever possible.

Finding the *truly optimal* set of multiplications is NP-complete, though, and I'm not sure if there is a good way to precompute and tabulate it like we do for a single exponent. But we could certainly compute a near-optimal set of multiplications using a variant of the power-by-squaring algorithm.

You would also have to be careful to identify when a given variable `x` refers to the same value at multiple points in the code, which is a bit hard to do at the macro level where types are unknown (so we can't be certain which operations might mutate `x`).