Garbage collection progress delegate

[Craig Black]

> I don't see how. Any estimate made before the GC starts would be very 
> crude.
> Probably no better than assuming that it'll take as long as it did last 
> time,
> and you can do that yourself.

A crude estimate is better than none at all.

> Estimates partway through the collection might be more accurate, but not 
> very
> useful, since you probably can't do very much with them without 
> potentially
> invalidating all the work the GC has done so far.

Indeed, you would have to be careful to not allocate memory on the heap. 
Beyond this, I don't think there would be any limitations.  There are plenty 
of things that you can do to provide feedback without allocating memory.

-Craig