Function spiral

spiral(smoothness, zoom): IterableIterator<Coord & {
step: number;
}>
Produces an Archimedean spiral. It's a generator.
```
const s = spiral(0.1, 1);
for (const coord of s) {
 // Use Polar coord...
 if (coord.step === 1000) break; // Stop after 1000 iterations
}
```
Parameters
- smoothness: number
  0.1 pretty rounded, at around 5 it starts breaking down
- zoom: number
  At smoothness 0.1, zoom starting at 1 is OK
Returns IterableIterator<Coord & {
step: number;
}>