Add reference counting to the VM functions

Add reference counting to the VM functions

This reduces memory pressure, keep engine->functions small and thus makes back
trace lookup faster. It became visible for example in the QtQuickControls
auto-tests that use plenty of loaders and we ended up with 30k+ functions.

Change-Id: Iaa5981f44e1e49ad9417a50c1e6a74946090dd28
Reviewed-by: Lars Knoll <lars.knoll@digia.com>