The issue #3920 was fixed but the fundamental flaw of lack of stack
depth check along with fibers still remains, even though it's not
easy to cause the issue.  Use `MRB_GC_FIXED_ARENA` to avoid the issue
for workaround.

After this patch, `obj.to_enum` raises `ArgumentError` if the object
does not respond to the enumerating method. This is incompatible to
CRuby but I think this behavior is better and CRuby should be updated
to behave like this.