gradient.rs: Refactor a for-loop to iterator combinators chain.

This comes down to personal preference probably. Would understand if it was not accepted due to preference of for-loops over combinators.

Note that .for_each was stabilized in rustc 1.21 which means it would bump the current req of ructc from 1.20. Although the docs already mistakenly point to 1.21.

If that's a problem .for_each is equivalent to .map().fold(|_| ()).