Merge: Read char

Merge: Read char

Semantics have been changed, before this PR, read_char was used to read a single byte and return -1 if EOF or timeout.

Since a Byte can have a value of -1, it is changed here:

* `read_char` now returns a Char (which, for the moment, is a byte, but this will change once UTF-8 is properly supported), and returns null on EOF or timeout
* `read_byte` has more or less the semantics of the old read_char, but returns null on EOF instead of -1 for the reasons mentioned earlier which could induce a wrong assumption about the state of the Stream

Edit: Still related to #1309

Pull-Request: #1334
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Jean Privat <jean@pryen.org>