runtime: use smaller fields for mspan.freeindex and nelems

runtime: use smaller fields for mspan.freeindex and nelems

mspan.freeindex and nelems can fit into uint16 for all possible
values. Use uint16 instead of uintptr.

Change-Id: Ifce20751e81d5022be1f6b5cbb5fbe4fd1728b1b
Reviewed-on: https://go-review.googlesource.com/c/go/+/451359
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>