Enable heap profiling with the non-moving collector

Motivation

Currently heap profiling isn't supported with the non-moving collector. But it would be quite helpful to have this.

Proposal

I think the main thing that is needed is for the heap profiler to be taught to traverse non-moving segments. I think the logic for segments just after a major gc is something like the following:

• Full: every block is live
• Active: marked blocks are live
• Current: marked blocks are live as well as everything before the allocation pointer.

We also need to be careful that it traverses large objects that live on the non-moving heap.

I think I could implement this.

I'm not quite sure if concurrent collection would have to be disabled if heap profiling is enabled. As I understand it, when concurrent collection is enabled, the non-moving gc only gives us a conservative view of what is alive on the heap. Perhaps that's ok, or perhaps we want to disable concurrent collection when heap profiling is enabled.