Implementation for r_set_equal()

This PR is a WIP for adding the implementation of `r_set_equal()` along with
the internal helpers and therefor targets #174 .

@neithernut I would like to have some pointers on how to approach this.
I don't think I have to do a complex operation where I loop in O(n^2)
trough all ht buckets and inside that in O(n^2) through all avl nodes,
and again, inside that one in O(n^2) through all linked lists? Seems
really much to complex for me to be the way how this has to be solved...
It's only used once and would make it only more difficult to maintain. I won't do that.