In GjsMaybeOwned<T> the JS::Heap<T> and JS::PersistentRooted<T>* members
are mutually exclusive. The m_rooted flag determines which one we are
using. Therefore, we can put the two in a union to save space.

The only thing we have to do is watch out when switching from one mode to
the other. When switching to rooted, we have to be careful to call the
destructor of JS::Heap, and when switching to unrooted we have to use
placement-new to construct a JS::Heap in the location of the union.

This reduces the size of ObjectInstance to 88 bytes on x86_64 (although
when the GjsMaybeOwned is in rooted mode, that is, the ObjectInstance is
in toggle ref mode and a GObject owns a reference to it, there are 32
"hidden" bytes allocated for the PersistentRooted.)