Added a fluid api for platform-specific coalescing

In order to test the limits of generics on iOS applications, and to do
some warmup fluent API design, I reimaged the Device.getPortable(1, 2,
3) in the form of a fluent platform coalescing API:

Get.Value(of: 0)
.Unless()
.iOS(requires: 1)
.Android(requires: 2)
.Windows(requires: 3);

Because I use my previous work, this code can be tested in a way that is
platform agnostic.