Realm: Report fragmentation statistics

[manopapad]

Currently on OOM we can print out the base pointers from instances, plus their sizes, to inspect the use of the memory space, to check if fragmentation is an issue. However, this is not complete because Realm doesn't expose the base pointer of the memory (presumably because this doesn't make sense for all memories).

An alternative that has been floated is to have Realm report stats on fragmentation. I don't know the full set of stats that might be useful to report, but one that would certainly help is to report the largest "hole", so the user knows what is the max allocation that could have succeeded.

[lightsighter]

I feel like we could do this pretty easily by an additional field to the InstanceAllocResult profiling response that would report the size of the largest hole that could be satisfied if success=false.
https://gitlab.com/StanfordLegion/legion/-/blob/master/runtime/realm/profiling.h?ref_type=heads#L249-253

[apryakhin]

I put the "reporting" in the backlog for now as it's behind "other items" that we planned to do first.