stdlib/strings: Removed module string, splitted in text and flat for flat strings

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
stdlib/strings: Added support for ropes in the new String model

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
stdlib/text: Added test for the methods of Text and its subclasses

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