Add measure of fragmentation to mem stats

[Evan Kirshenbaum]

[imported from HPE issue 226]

It would be useful if the GC mem stats gave an indication of fragmentation. Two easy measures would be (1) the size of the largest block found in sweep and (2) counts of number of blocks found at each size level (e.g., an array matching the global allocator, added to only by sweeping threads).

And maybe also a histogram of sizes of objects found during tracing.