* add CloseRead/CloseWrite on streams
This changes the behavior of `Close` to behave as one would expect: it closes
the stream. The new methods, CloseWrite/CloseRead allow for closing the stream in
a single direction.
Note: This _does not_ implement CancelWrite/CancelRead as our stream muxer
_protocols_ don't support that.
fixes#9
* remove stream util helpers
FullClose and AwaitEOF were introduced to work around the fact that calling
Close on a stream only closed the write half. All users must adapt their code
to the new interfaces, so this change is intentionally breaking.
