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>

src/runtime/export_test.go

@@ -337,7 +337,7 @@ func ReadMemStatsSlow() (base, slow MemStats) {
 			slow.BySize[i].Frees = bySize[i].Frees
 		}
 
-		for i := mheap_.scav.iter(); i.valid(); i = i.next() {
+		for i := mheap_.scav.start(); i.valid(); i = i.next() {
 			slow.HeapReleased += uint64(i.span().released())
 		}