lib/standard/ropes: WIP Ropes refactoring (yet again)

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/ropes: Introducing Leaf, a buffered FlatString with a fixed size

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
benchmarks: Benches adapted to new Ropes

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