Implement Stream{Source, Sink} for T: Mem{Source,Sink} and add super trait bound

[LukasKalbertodt]

A Mem{Source, Sink} is worth more than a Stream{SourceSink}, as in: can always do more. It would be really useful to:

• Add the : Stream* super trait bound to Mem* traits
• Add a default implementation of Stream* for all types T: Mem* (this implementation would likely be similar to PLYs impl: many function pointers)

This would make it possible to easily transfer data from one MemSource to a MemSink.