Open Radar

 
The most common implementation of an object that conforms to `Generate` is so generic, I can’t help but feel that it should be included in the standard library. As far as I can tell, this implementation should work 90% of the time:

```
struct CollectionGenerator<T>: Generator {
    var items: Slice<T>

    mutating func next() -> T? {
        if items.isEmpty { return nil }
        let ret = items[0]
        items = items[1..items.count]
        return ret
    }
}
```

This code is ripped almost verbatim from the Advanced Swift session video.

Steps to Reproduce:
When implementing `Sequence`, look for a basic object to use that conforms to `Generate`.

Expected Results:
An object is provided that handles the most common use case

Actual Results:
An object can’t be found, leading me to writing (or duplicating) code.