Merge: lib/core: Renamed `Text::bytelen` to `Text::byte_length`

As one famous man once said: "Je vous ai compris!".

So, I understood that some of us (@xymus, @ppepos, @BlackMinou, etc.) did not like the `bytelen` name, since it was not that explicit.
Well, its days are numbered and it is replaced by the longer but more explicit `byte_length`.

Pull-Request: #2173
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>