runtime: move internal GC statistics from memstats to gcController

runtime: move internal GC statistics from memstats to gcController

This change moves certain important but internal-only GC statistics from
memstats into gcController. These statistics are mainly used in pacing
the GC, so it makes sense to keep them in the pacer's state.

This CL was mostly generated via

rf '
    ex . {
	memstats.gc_trigger -> gcController.trigger
	memstats.triggerRatio -> gcController.triggerRatio
	memstats.heap_marked -> gcController.heapMarked
	memstats.heap_live -> gcController.heapLive
	memstats.heap_scan -> gcController.heapScan
    }
'

except for a few special cases, like updating names in comments and when
these fields are used within gcControllerState methods (at which point
they're accessed through the reciever).

For #44167.

Change-Id: I6bd1602585aeeb80818ded24c07d8e6fec992b93
Reviewed-on: https://go-review.googlesource.com/c/go/+/306598
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>