equalTo() matcher to avoid circular references

[tomhaines432]

requirement: Stop the equalTo() matcher from traversing any object it has already traversed during the match.

driver: This will stop it from overflowing the stack when an object being checked contains a circular reference.

[oneronaut]

This could potentially be solved by keeping track of every object traversed and never checking the structure of it again, once it has been traversed. This will stop any kind of circular reference from overflowing the stack during equalTo() matching.