runtime: split spans during allocation without treap removal

Now that the treap is first-fit, we can make a nice optimization.
Mainly, since we know that span splitting doesn't modify the relative
position of a span in a treap, we can actually modify a span in-place
on the treap. The only caveat is that we need to update the relevant
metadata.

To enable this optimization, this change introduces a mutate method on
the iterator which takes a callback that is passed the iterator's span.
The method records some properties of the span before it calls into the
callback and then uses those records to see what changed and update
treap metadata appropriately.

Change-Id: I74f7d2ee172800828434ba0194d3d78d3942acf2
Reviewed-on: https://go-review.googlesource.com/c/go/+/174879


Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>