Add a `TransportStream` trait for transport-layer implementations
Add a `TransportStream` trait for transport-layer implementations

Any class that provides a transport layer implementation that HTTP/2
can use should implement this trait.

For now, it is only an extension of the `old_io::Stream` trait and
does not require any additional methods to be implemented.

jinnjuice/solicit,stepancheg/solicit,tempbottle/solicit,gwicke/solicit,Galvanization/solicit,mlalic/solicit