Merge: Added `copy_from` service to `NativeString`

Usability PR, since I got tired of handling indexes and different structures manually, this method abstracts the job of copying the content of a `Text` to a `NativeString` object.

Pull-Request: #2062
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>

diff --cc lib/core/text/abstract_text.nit
Simple merge

diff --cc lib/core/text/flat.nit
Simple merge