stdlib/strings: Introduced class Text, will be used as a new abstraction between Flat Strings and Ropes.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
stdlib/strings: Changed test_isa to be compatible later on when Strings are independant from Collection.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
stdlib/strings: Detached the Text block from Collection.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
stdlib/strings: Cleaned the old way of defining private attributes (readable var _...) and replaced with normal vars.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
stdlib/strings: Relocalized Text in its own module, string.nit contains the Flat representation (name is for legacy puposes).

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