SE-0374: Add `sleep(for:)` to Clock

[Karl]

@kiel

During the pitch, this comment was made:

@Philippe_Hausler

Perhaps that is my bias against the performance impacts of existentials over concrete generic values

Perhaps what follows is reading a bit too much into the above comment. While I can't imagine how else it might work, could there be a way of avoiding the existential (that is, use a concrete type only) whose behaviour can be controlled environmentally, much like how a Task can inherit priority, could it inherit a clock or the clock inherit a way to control the passage of time?

Existentials aren't so bad - especially now that we can use the language's actual existentials for PATs. I've found that the compiler is quite good at devirtualising them, so they can perform identically to generics in many situations.

For something like this where you're going to be suspending for some appreciable amount of time anyway, I think the overhead of using an existential is going to be negligible. And if you do find it to be significant, you can still opt for a restricted set of concrete types by defining an enum. In any case, this function would still be a convenient addition.