Early abort for forEach, peek multiple

Adds the capability to peek at multiple entries at the head of the
queue. I didn't have to change QueueFile, as its forEach already can
avoid reading bytes, but this optimization to abort early saves some
object creation.

Not a breaking API change.
What do you think about making this an interface, naming it `ElementVisitor`, and deprecating the old one and corresponding `forEach`? No reason to keep both around.