Better handling of powers in modcc. (#2061)

- $x^{-1} \Rightarrow 1/x\quad \forall x$
- $x^n \Rightarrow x\cdot \dots \cdot x \quad x\in N; |x| < 5$
- $x^n \Rightarrow 1/(x\cdot \dots \cdot x) \quad x\in N; |x| < 5; x <
0$
- $b^e \Rightarrow \exp(\log(b) e)\quad \forall  b, e$

The last point introduces potential errors when `pow(b, e)` is allowed,
but `log(b)` is
undefined. These occur exactly when all of the following is true
- $b < 0$
- $e\in N$
- $e$, $b$ not known at compile time (since we cover these cases before)