RGB <-> LabSpace memoization

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

I might be wrong here, but try this. I think instead of declaring `pixelToLAB` separately you could write this line as follows:

```swift
let memoizedRGBToLab = memoize { IN_RGBToLAB($0.toRGBVector()) }
```