Test if a type conforms to a non-existential protocol

[Jumhyn (Frederick Kellison-Linn)]

@anandabits

I noticed @_implementationOnly in your code. I haven't run into that before. I can guess what it might do, but would like to understand it better. Can you explain or point me in the right direction to learn more?

Not @Nevin, but the relevant thread is here!