lib/standard/string: Bugfix, forgot iterator on index_of_from

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/string: Added implementations for to_upper and to_lower on FlatBuffer

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
Merge: String bugfixes

An iter.next was forgotten when implementing methods in string.nit, also to_lower and to_upper were not present in FlatBuffer.

This fixes both issues.

Pull-Request: #435
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>