These tests currently fail by overflowing the stack.
Value comparisons are drawn from what Chrome
and Firefox do, which doesn't appear to be in the
spec?

This test is likely to be broken by the next change

We can use the existing RecursionLimiter type
used by GcObject's Debug impl for this purpose.
We just need to refactor it to allow both liveness
and visitation tracking, using a HashMap of ptrs
to states instead of a HashSet of ptrs.

Use the newly refactored RecursionLimiter to
check for recursion, and limit it. Throw a TypeError
as mandated by the spec.

Use the new RecursionLimiter type to prevent
overflows from conversions in ordinary_to_primitive.
The spec doesn't say what to do here, so we follow
v8 / SpiderMonkey in returning a default value for
the type hint -- either 0. or "". More details in
the method documentation.

Someone added `as_gc_object` right as I added
`add_gcobject`. What are the chances? Switched
to the new method.