runtime: process ptr bitmaps one word at a time

runtime: process ptr bitmaps one word at a time

Read the bitmaps one uintptr at a time instead of one byte at a time.

Performance so far:
 Allocation heavy, no retention: ~30% faster in heapBitsSetType
 Scan heavy, ~no allocation: ~even in scanobject

Change-Id: I40d492b50d7f89d1b4261c2de58f6d255fa5e93e
Reviewed-on: https://go-review.googlesource.com/c/go/+/407036
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>