runcontainer: make fewer memory allocations in iandNotRunContainer16().

There is no need to convert both run containers to bitmap
containers first. There already is an efficient implementation
of andNot in AndNotRunContainer16(), so use it.

runcontainer.go

@@ -2464,12 +2464,7 @@ func (rc *runContainer16) toBitmapContainer() *bitmapContainer {
 }
 
 func (rc *runContainer16) iandNotRunContainer16(x2 *runContainer16) container {
-	rcb := rc.toBitmapContainer()
-	x2b := x2.toBitmapContainer()
-	rcb.iandNotBitmapSurely(x2b)
-	// TODO: check size and optimize the return value
-	// TODO: is inplace modification really required? If not, elide the copy.
-	rc2 := newRunContainer16FromBitmapContainer(rcb)
+	rc2 := rc.AndNotRunContainer16(x2)
 	*rc = *rc2
 	return rc
 }