Currently, we map and grow the heap a whole arena (64MB) at a time.
Unfortunately, in order to fix #32828, we need to switch from
scavenging inline with allocation back to scavenging on heap growth,
but heap-growth scavenging happens in large jumps because we grow the
heap in large jumps.

In order to prepare for better heap-growth scavenging, this CL
separates mapping more space for the heap from actually "growing" it
(tracking the new space with spans). Instead, growing the heap keeps
track of the "current arena" it's growing into. It track that with new
spans as needed, and only maps more arena space when the current arena
is inadequate. The effect to the user is the same, but this will let
us scavenge on much smaller increments of heap growth.

There are two slightly subtleties to this change:

1. If an allocation requires mapping a new arena and that new arena
   isn't contiguous with the current arena, we don't want to lose the
   unused space in the current arena, so we have to immediately track
   that with a span.

2. The mapped space must be accounted as released and idle, even
   though it isn't actually tracked in a span.

For #32828, since this makes heap-growth scavenging far more
effective, especially at small heap sizes. For example, this change is
necessary for TestPhysicalMemoryUtilization to pass once we remove
inline scavenging.

Change-Id: I300e74a0534062467e4ce91cdc3508e5ef9aa73a
Reviewed-on: https://go-review.googlesource.com/c/go/+/189957
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>