Circular data & cross references #110

Adding an option (checkDuplicates) to catch circular and cross
references, disabled by default so as not to slow down any existing code (only a few bits of code are still executed when the option is disabled).

Implemented following @puzrin suggestion that is: do a first pass to get the duplicates, and then put the aliases inside the algorithm. Slower than @larkox 's implementation, but perhaps easier to maintain.

Also added a related test.

NB: There may be some unused aliases if a duplicate is found inside a custom type, which must be a pretty rare case.
I could agree with you if there were actual Set data structure in JavaScript. In current state it looks misleading, so it would be cool, if you change this to the array version.