Introduce Any<P> as a better way of writing existentials

[DevAndArtist (Adrian Meister)]

Cross posting this as I think it related to this thread as well:

Lifting the "Self or associated type" constraint on existentials

I do agree that we should allow implicit opening of existentials when they're passed as unique generic arguments to a generic function; I agree that will likely cover the majority of places you might want "self-conformance". I don't think banning existentials as function arguments is necessary, or really possible to begin with, since existential types still can be bound to generic type arguments. To work with an Array<Protocol>, Array's generic methods need to be able to work in terms of the Pro…