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* add a context to MuxedConn.OpenStream

* add a context to Conn.NewStream

* 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.