Issue Description:

I noticed that if you have a partial range and you call relative(to🙂 with a collection that does not contain the range's specified bound you get a fatalError

This is a simple playground that shows the problem:

let emptyCollection = [Int]()
let nonEmptyCollection = (0..<20).map { $0 }
let arraySlice = nonEmptyCollection[5...]

let partialRangeFrom = 1...
let partialRangeUpTo = ..<1
let partialRangeThrough = ...1

partialRangeFrom.relative(to: emptyCollection)
partialRangeUpTo.relative(to: arraySlice)
partialRangeThrough.relative(to: arraySlice)

The playground will crash on the first relative(to🙂 call but if you comment that out it crashes on the next and so on.

I have found a simple fix for the issue and am submitting a pull request with the change and tests that would crash as it is now.