Put consumed chunk errors back in their channels
Declare and export channel errors
muxer.go -> protocol.go
Limit sequence numbers to 21 bits on the protocol level

21-bit integers fit neatly in 3 bytes when encoded as varints.
Begin the process of replacing Connection with Transport
Transport methods now take channel instances instead of sequence numbers
Transport -> Stream
Clean up channel.go
Remove the now redundant Transport type
Define ErrStreamClosed