runtime: separate scavenged spans

runtime: separate scavenged spans

This change adds a new treap to mheap which contains scavenged (i.e.
its physical pages were returned to the OS) spans.

As of this change, spans may no longer be partially scavenged.

For #14045.

Change-Id: I0d428a255c6d3f710b9214b378f841b997df0993
Reviewed-on: https://go-review.googlesource.com/c/139298
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>