String-dedup test failures

[Roman Kennke]

I think we need to clean up the string dedup table *after marking*. If a 
byte[] is not reachable after marking, we need to clean all references 
to it, because *it may become trashed* during evac. If we want to keep 
it alive, then we need to mark it. Evac'ing an unmarked byte[] after 
marking is calling for troubles, because we probably access a pointer 
that is about to be invalidated.

Roman