multi-rope; input order working
support RLO in mrope

It is somehow surprising that on insertion, mrope is simpler than rope. The BCR
paper actually describes the algorithm in a way similar to mrope.
the multiple version works on toy, but not MT

will be hard to debug...
fixed a typo in mrope_multi; update main.c
removed single-rope string insertion

All the following changes will be in mrope only. Mrope is cleaner and opens the
possibility of multi-threading (to be implemented).