What is the correct behavior of `Stdin.readByte`?

[Josh Goebel]

Working on new File IO abstractions and found this TODO issue:

if (__buffered != null) {
        // TODO: Is this correct for readByte()?
        // Emit the last remaining bytes.
        var result = __buffered
        __buffered = null
        __waitingFiber.transfer(result)

This behavior is clearly incorrect. If the input closes then the last call to readByte will return a string with all remaining buffered data (or an empty string), not a byte. This begs the question of what SHOULD happen? I would suggest that repeat calls to readByte should continue to return bytes until either null (is returned repeatedly) or a Fiber.abort is triggered.

This is the same question as with readLine on whether reaching the EOF should be an error condition or not.

Related: #125