runtime: shrink stacks in parallel Shrinkstack does not touch normal heap anymore, so we can shink stacks concurrently with marking.
runtime: shrink stacks in parallel
Shrinkstack does not touch normal heap anymore,
so we can shink stacks concurrently with marking.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, khr, rlh, rsc
https://codereview.appspot.com/122130043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go