Added specialized MessageNodes

- a monomorphic message node is the first specialization and caches the invokable of the first message send
- a polymorphic message node caches 8 elements
- the megamorphic node is used if there are more than 8 receiver classes encountered, it does the complete lookup and send and is the most general state
[TODO: not yet properly working, there are issues with the Bounce and Queen benchmarks]

Signed-off-by: Stefan Marr <git@stefan-marr.de>