Merge: Fix virtual types in string

`SELFTYPE` in String is used to dintinguish buffers from strings. Unfortunately it is sometime badly used.

This series try to at least improve thing so that operations on buffers do not crash at runtime.

Still, I think there is something fishy in the String vs Buffer API

Pull-Request: #1133
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>