RGB <-> LabSpace memoization

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

Might have to do this actually, since types can't be inferred here.

```swift
let memoizedRGBToLAB: RGBAPixel -> INVector3 = memoize { IN_RGBToLAB($0.toRGBVector()) }
```