Change UnwrapCUE() to Underlying()

[sdboyer]

It doesn't feel like there's an ideal name here, but UnwrapCUE() is using "wrap" in a way that just doesn't feel right.

Also, i don't like staring at CUE all over my code. It's yelly.

Underlying() seems better, and vaguely reminiscent of Go's notion of the type underlying an interface, which at least feels less wrong than Unwrap. Open to suggestions, though. For the limited window before i just do this :)

@IfSentient this change is coming, and it's obviously breaking. It'll be a trivial refactor though, just a method rename.