Merge: Copy to native

Added copy_to_native method in Text, also its optimized versions for subclasses

As a bonus (and because it simplified my task here), substrings now return FlatTexts only instead of simple Text.

The latter will permit the implementation of StringReader as BufferedReader later in an optimized way

Pull-Request: #1335
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Jean Privat <jean@pryen.org>