[ci skip] [WIP] 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...
Also: the intention of this double loop-construct is to check whether all elements of one set are in the other, too. This, however, doesn't neccessarily mean equality. It means nothing but A being the subset of B.
You either have to do it the other way around too (ugh), or verify that the number of elements are equal, too.