All classes and modules in the system are reachable via the weak
subclasses collections on each class. This reduces overhead of
creating a new module, class or metaclass by avoiding the extra
runtime structures and reduces the overall memory footprint by
N classes * ConcurrentHashMap per-object memory plus WeakReference
memory and overhead.

These are only used for one method cache: initialize. This method
is only used for constructing new objects of this class's type, so
this is wasted overhead for everything except concrete classes.

The patch here initializes it lazily since many (most?) RubyModule
subclasses will never need it. We don't bother making it volatile
or atomic since there's no interesting state (occasional double
init will hurt nothing).

Worth pointing out that with indy `new` logic, we may also never
need this cache. The first call may be slow path in indy sites,
but so is the first init of this cache and the first call to use
it. A future commit may remove this altogether.

Eagerly initializing these adds memory churn to the allocation of
many RubyModule types that will never even access these fields.
Instead we lazily initialize them using an atomic update.

Hardly any RubyModule instances will ever use these fields, so
punt them into a separate data object.