This Pull Request fixes/closes #608.

It changes the following:

• Adds RecursionLimiter. This tracks the GcObject pointers that have been visited during a Debug::fmt call.
• Adds a test for the stack overflow.

The same object cannot appear twice in the output, by design. JS object graphs tend the be huge, and comprehending a massive data dump is nearly impossible. If the same object appears twice, even if not recursive, it is always elided. The following graph:

let a = {};
a.c = a;
let b = {
 d: a, e: a
};
debug(b);

Would print:

{
  d: {
    c: ... // Recursion protected
  },
  e: ... // Will never appear twice
}