runtime: implement GC scan work estimator

runtime: implement GC scan work estimator

This implements tracking the scan work ratio of a GC cycle and using
this to estimate the scan work that will be required by the next GC
cycle. Currently this estimate is unused; it will be used to drive
mutator assists.

Change-Id: I8685b59d89cf1d83eddfc9b30d84da4e3a7f4b72
Reviewed-on: https://go-review.googlesource.com/8833
Reviewed-by: Rick Hudson <rlh@golang.org>