Replaced most of the usages of parallel_for with that of parallel_for_.

This should allow many algorithms to take advantage of more parallelization
technologies.
Split CLAHE into its own file, because it's faster that way.

Yes, it's as ludicrous as it sounds, but it's still true. Bizarrely,
the previous commit makes CLAHE run about 10% slower on Android, even
though it doesn't even touch any CLAHE code. Splitting it off fixes that,
although the reason it does is a mystery for the ages.

It's cleaner when it's in its own file, anyway. ;=]
Merge pull request #934 from SpecLad:parallel-for