This computes the maximum possible waste in a size class due to both
internal and external fragmentation as a percent of the span size.
This parallels the reasoning about overhead in the comment at the top
of mksizeclasses.go and confirms that comment's assertion that (except
for the few smallest size classes), none of the size classes have
worst-case internal and external fragmentation simultaneously.

Change-Id: Idb66fe6c241d56f33d391831d4cd5a626955562b
Reviewed-on: https://go-review.googlesource.com/49370
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>