Noncopyable Existentials and Casting

After talking with some people who were trying to use Noncopyable existentials for various purposes, I set out about a month ago to see what was possible here. I have since implemented an experimental feature "NoncopyableCasting" that enables many kinds of common operations involving noncopyables. I'm still exploring what's feasible, but here's a partial list of what you can do now with this feature: type(of:) borrows its argument instead of consuming. Most forms of is borrow the subject ins...