gocore: distinguish released memory
gocore: distinguish released memory

Split up the free span space into 2 categories, the space which we
have released back to the OS vs the space we haven't.

Change-Id: I897c57a10e5cb4ab7321711fece8addcf5f4ab19
Reviewed-on: https://go-review.googlesource.com/92417
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

golang/debug