stdlib: Modified strings for performances with substring operations

Signed-off-by: Lucas BAJOLET <r4pass@hotmail.com>
stdlib: Strings, modified concatenation operations (avoids creating a useless buffer)

Signed-off-by: Lucas BAJOLET <r4pass@hotmail.com>
stdlib: Ropes, added first draft, fully functional

Signed-off-by: Lucas BAJOLET <r4pass@hotmail.com>
stdlib: Corrected comments for function has_substring in Strings

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