[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...
In the header of avl_equal you have this attributed as "nonnull". This condition doesn't make sense to me in this case.