lib/standard/ropes: Added test for Ropes.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/text: Added global test on all the API of Text and subclasses.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/string: SELFTYPE now refers to String instead of its implementations.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/string: Added methods prepend, append and insert_at to String interface and implementations.

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