Add fragmentation statistics to GHC.Stats

Motivation

Fragmentation is a key statistic when using the non-moving garbage collector. The event log has information about heap fragmentation, but it would be helpful to have the same information available from GHC.Stats.

As far as I understand we can already get at this statistic by taking gcdetails_mem_in_use_bytes - gcdetails_live_bytes . But I'm not sure that's right. And even if that's true, I think it would be more user friendly to make this stat available in an explicit way.

Proposal

Add a field to GHC.Stats.GCDetails called gcdetails_fragmentation_bytes that gives the same stats as the event log fragmentation counter.

I'm happy to implement this.