Evaluate API for random(inRange:) #89

From a TODO in UnsignedInteger+Extensions: There is a public function random that takes in a CountableClosedRange of integers and returns a random int from within the range. The TODO comment suggests that this function could live as a member function on CountableClosedRange. So it would look something like:

(0..<10).ip_random()

I think to address this issue we need to:

1. Start a discussion about whether we like this idea.
2. Try switching the code to use the new API, with updated test coverage.

Not necessarily in that order. I think taking a stab at (2) could help inform (1).