For APIs, strings to insert need to be reversed

This makes the APIs less intuitive, but simplifies coding. This especially
helps the radix algorithm I am going to implement.
inserting multiple strings in RLO; NOT WORKING!!!

Only implemented the sketch. Definitely not working.
simplified single-string RLO
added CLI for insert_multi(); NOT WORKING YET!!!
code backup
updated gitignore
work on toy examples, but not on MT
the algorithm works, but there is a bug...

probably in rle again. We can insert a run base by base, not together.
bugfix: inserting long runs not working

fortunately, the bug is pretty easy to fix
reduced wasteful rank2a() calls
bugfix: not working for variable-length input

rope_insert_multi() is slightly faster than the single-sequence version.
Further improvement possible, but complicated.
expose the rope struct
no change at all
as rpitr_t is exposed, don't allocate for rpitr_t
renaming a variable
simplified code a little bit
added RCLO in the single-string version