This change modifies the treap implementation to support holding all
spans in a single treap, instead of keeping them all in separate treaps.

This improves ergonomics for nearly all treap-related callsites.
With that said, iteration is now more expensive, but it never occurs on
the fast path, only on scavenging-related paths.

This change opens up the opportunity for further optimizations, such as
splitting spans without treap removal (taking treap removal off the span
allocator's critical path) as well as improvements to treap iteration
(building linked lists for each iteration type and managing them on
insert/removal, since those operations should be less frequent).

For #30333.

Change-Id: I3dac97afd3682a37fda09ae8656a770e1369d0a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/174398
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>