Fix for JSON.stringify with objects
Fix for JSON.stringify with objects

Infinite recursion bug in `pseudoToNative`:
* was attempting to output non-enumerable 'constructor' property
* cycles in 'constructor' or any other prop would hang

Strips non-enumerable properties from the output, and adds a
fix for cycles in the `pseudoToNative` converter.

Actually attempting to `JSON.stringify()` a cycle graph now throws
an actual error.

Fixes https://github.com/NeilFraser/JS-Interpreter/issues/106

arcatdmz/JS-Interpreter,NeilFraser/JS-Interpreter,arcatdmz/JS-Interpreter,NeilFraser/JS-Interpreter,NeilFraser/JS-Interpreter,arcatdmz/JS-Interpreter