runtime: make mTreap iterator bidirectional

This change makes mTreap's iterator type, treapIter, bidirectional
instead of unidirectional. This change helps support moving the find
operation on a treap to return an iterator instead of a treapNode, in
order to hide the details of the treap when accessing elements.

For #28479.

Change-Id: I5dbea4fd4fb9bede6e81bfd089f2368886f98943
Reviewed-on: https://go-review.googlesource.com/c/156918


Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>