handle reference cycles in is_deep_equal to avoid stack overflow

[bandaloo]

i was scratching my head, thinking if it's just a "garbage in, garbage out" situation and it's best to ignore it. but i looked at some other assertion frameworks to see if they handle this sort of thing, and they typically do. since the code changes are minimal, and the extra memory it uses to keep track of visited tables is bounded by the size of the tables being compared, i feel like it's worth it.

the first test i added actually passes in the original is_deep_equal implementation, but i thought it is just a good test to have. the last two tests would go into infinite recursion without this fix.

[EvandroLG]

Another really good catch, @bandaloo! Thanks a million, man!