Improve font locking performance on strings with escaped characters

[yuhan0]

Fixes #589.

I'm not entirely sure what the original code was trying to achieve, but it was behaving pathologically on strings with lots of escaped characters (probably due to re-search-backward), so I reimplemented it in a more straightforward manner.

All tests are passing so it doesn't seem to have broken anything :)

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[bbatsov]

Thanks!