lib/standard/ropes: Added cache on indexed access.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/ropes: + operation in O(1) time now

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/bufferized_ropes: Force insert_at for + operation

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
Merge branch 'ropes_optimization' into ropes_everywhere