Use for loops instead of forEach for speed

```
~/src/turf-within〉node bench.js
turf-within x 392,577 ops/sec ±0.57% (98 runs sampled)
turf-within.fast x 679,989 ops/sec ±0.55% (98 runs sampled)
```

This has old code still in it for comparison purposes - needs a double-tap before merge if it's worth while.