Use a really unfancy algo for positionForCharacterIndex

It’s faster than the skip list. 20% in my tests.
:racehorse: This is _considerably_ faster.

Creating 240 markers with literal range syntax and with Range objects.

Before (note the 4.85ms):

![use point and range](https://cloud.githubusercontent.com/assets/69169/3939920/7daf04f8-24d0-11e4-9a0d-9b8cdd87f43b.png)

Before (note that `use-range` and without are the same):

![optimize range](https://cloud.githubusercontent.com/assets/69169/3939966/83850264-24d1-11e4-81f0-43fc2ed39dc0.png)
Don’t use splat. It’s slow and unnecessary.