Reconsider names of the std::comm types

[brson]

The name stream has never felt quite right to me for this function. While it is a descriptive name, it is also generic and heavily overloaded. This is the interface to Rust concurrency, and it will be in the prelude, so it should have a distinct name that we're happy with everybody using forever.

Edit: see comments below. A lot of the names in this module are unsatisfying.

[brson]

Also rename any associated types

[bblum]

It could well be called "pipe".

[Aatch]

@brson are this and #5301 still relevant? With the new scheduler, we can rename them, since they are already being re-written (though it appears that the current re-implementation shares the naming, is this deliberate?)

[brson]

Yeah, this is still relevant. We've kept the old names for compatibility. I'm going to update the title to reflect that basically every name in std::comm needs to be reconsidered, GenericFoo, SmartFoo, MegaPipe, oneshot. It's just not cohesive.

[alexcrichton]

They have been reconsidered! Only Chan, Port, and SharedChan remain.