perf: Consider something like `pow2k`

Both `Invert` and `Pow22523` repeatedly square in a loop. The overhead of repeatedly calling Square (and having to shuffling data in/out of registers) adds up to a decent chunk of execution time.

Doing something like `func (v *Element) pow2k(x *Element, k uint)` with the precondition that `k >= 1`, dramatically improves performance of the two operations like thus:

```
Invert-4 11.3µs ± 0% 7.1µs ± 0% -37.33%
Pow22523-4 11.1µs ± 0% 7.0µs ± 0% -37.32%
```

Numbers taken with `purego`, but the amd64 assembly implementation will also benefit (and can be written without having to spill to the stack at all).