Combine GenericChan and GenericSmartChan traits

[brson]

GenericSmartChan contains try_recv which returns None if the sender has closed. I split it out from GenericChan because I didn't think there were reasonable semantics for Readers that want to implement try_recv, but I think in the case of I/O streams try_recv can basically mean 'receive unless EOF`. Does this make sense?

[brson]

This is actually nonsense. I was confusing ports and chans. GenericPort already contains both recv and try_recv.