Consider never returning EOF

[bddap]

https://noiseprotocol.org/noise.html#application-responsibilities states Session termination needs to be handled by the application. An eof on the stream should not be interpreted as some other error.

Option 1: Encstream implements In eof signaling.

Option 2: Encstream delegates eof signaling to the user.

Option 1 is likely nicer for the user.