lib/standard/string: Fixed bug in to_s implementations in NativeString

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/string: Added redef for substrings in String

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/ropes: Removed ropes.nit from standard library

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/ropes: Rewrote ropes base block.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/ropes: Added buffered variant of Ropes.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/ropes: Introducing RopeBuffer, a mutable `Rope` optimized for concatenations.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib: Updated lib for compliance with the new ropes

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
tests: Updated tests for new Ropes

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard: Re-activate ropes in standard

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
benchmarks/strings: Adapted for new Ropes.

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