Auto merge of #12 - Ogeon:gradient, r=Ogeon
Auto merge of #12 - Ogeon:gradient, r=Ogeon

Implement Gradient slicing and exact size iteration

This adds some missing features to the `Gradient` type:

 * Implement `ExactSizeIterator` for `Take`.
 * Make it possible to slice `Gradient`, and thereby close #4.

Ogeon/palette,Ogeon/palette