RGB <-> LabSpace memoization

PR started to address issue #9 . This is work in progress for comments and direction.

Formalizing our offline discussion:

Let's just not hash the vectors since hashing floating point values is a world of hurt. Easiest way to go is to implement a private function (called `pixelToLABVector`, perhaps) that converts `RGBAPixel` to an `INVector3` and returns the output of passing that vector to `IN_RGBToLAB`. Then `pixelToLABVector` can just be passed into `memoize()` and it will automatically use the `RGBAPixel` values as the cache keys (w/ simple integer hash above). We're only going to memoize RGB -> LAB conversion because the other way around has a negligible performance cost due to k=16 being the cluster size.