increase speed

Right now it seems like `highlight` is about 1/10 the speed of `grep`. I think it should be relatively easy to get to 1/2 the speed of grep.

#1 (add a benchmark) should probably be completed before attempting this.

Also, #3 (modify `childOf` function) maybe be some low-hanging fruit.

Start with issue #1 and its planned benchmark, then compare highlight's runtime with grep. Review the childOf function mentioned in issue #3 as a possible optimization area. Done means the benchmark shows highlight reaching roughly half of grep's speed without changing its highlighting behavior.

Written by the indexing model from the issue text.