lib/standard/string: Proper handling of trivial cases in substring_from.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/string: bugfix in Array::to_s when encoutering another structure than a FlatString

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
tests: Added test for the special case of Array.to_s with a Rope.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/string: reverse iterator fix on substring. Fixes #709

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