Fixes rightOf/leftOf misbehaving with newlines

[gampleman]

Fixes #65.

It replaces rightOf with an implementation based on String.indexes, which seems consistently much faster (about 10x in the N=1000 case) than the existing regex approach.

Interestingly, leftOf is faster using String.indexes with a low number of matches (for instance when tested with 10 matches), but becomes significantly slower as the number of matches increases (with a 1000 matches, it is much slower than the regex version).

Those are the results in Webkit. In Chromium the results look a bit different:

Same thing in Spidermonkey:

At the moment I'm thinking of replacing rightOf with the String.indexes based version, since in Webkit it's such a clear win, the performance in Chrome isn't terrible and well... basically nobody uses Firefox...

[miniBill]

[I use Firefox 😭]
I think that realistically, you wouldn't use rightOf/leftOf in a string with 10k matches? My expectation is that the median number of matches is 1, and the average probably not much higher, so switching to the indexes-based approach makes sense to me for both