runtime: add successor method to treap

runtime: add successor method to treap

This change adds a method for computing a treap node's successor
to the treap, which will simplify the implementation of algorithms
used for heap growth scavenging.

For #14045.

Change-Id: If2af3f2707dbcbef5fb6e42cb2712061f9da5129
Reviewed-on: https://go-review.googlesource.com/c/144718
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>