Possible minor perf bug in `work`

It looks like work can consume exactly all of the fuel and still complete the merge:

https://github.com/TimelyDataflow/differential-dataflow/blob/3d4bd15daf94bf4e5a40aaa7592778af6e2ed913/src/trace/implementations/vec.rs#L126

		if effort >= *fuel { *fuel = 0; }
		else 			   { *fuel -= effort; }

However, in the doc for Merger,

	/// If `fuel` is non-zero after the call, the merging is complete and
	/// one should call `done` to extract the merged results.
	fn work(&mut self, source1: &Output::Base, source2: &Output::Base, frontier: &Option<Vec<T>>, fuel: &mut usize);

Unless I'm overlooking something, this means we may notice that the merge is in fact complete only on the next invocation of work. If work finishes with exactly zero fuel, we should be able to call done right away.

Start at src/trace/implementations/vec.rs around line 126 and compare the fuel handling with the Merger::work documentation quoted in the issue. Trace when a merge is considered complete and when done may be called; done means the exact-fuel case follows the documented completion contract without requiring another work invocation.