Ok, I knew it was slow, but I didn't realize it was *that* slow. There's some benchmarks for these in coords_test.go:

```
$ go test -bench . buildblast/lib/coords
PASS
BenchmarkEveryOffset	     500	   3859029 ns/op
BenchmarkEachOffset	   10000	    168036 ns/op
BenchmarkForLoopOffset	   50000	     71102 ns/op
BenchmarkForLoopIndexOffset	   10000	    118457 ns/op
ok  	buildblast/lib/coords     9.498s
```

So while EveryOffset takes 4ms to iterate, EachOffset only takes 160us, which is pretty close to the plain for loop of 71us (that is, assuming you are doing anything at all in the loop).