Add skipWhile to Array

The opposite of takeWhile, skips all elements until the condition returns false,
returning the remaining elements.

[1, 2, 3, 4, 5, 2].skipWhile { $0 < 4 }
// → [4, 5, 2]
Add takeWhile and skipWhile tests

It'll be nice once these can actually build! Discovered some Swift bugs
writing these tests but eventually the descendingArray variable
shouldn't be needed. For now it's there to ensure this isn't just
filtering things which fail the condition.