stdlib/strings: Cleaned code for classes depending on Rope and Flat (definition of the char view).

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
stdlib/strings: Fix, added SELFTYPE as Rope for Rope classes, can cause bugs when using operators based on SELFTYPE.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
stdlib/text: Added method to_b on Text and subclasses, replaces new Buffer.from which is now private.

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