The goal here is to start splitting off the method reification,
which many (most) people don't need, so we can still get the
benefit of generating a real class for every custom Ruby class
without the hassle of binding a bunch of methods.

See #4643.

This is a first step toward unifying all forms of reification.

In this commit, all the hand-implemented RubyObjectVar* and
ObjectAllocator* are instead generated as needed and then reused
statically. Since this will be a bounded number of classes (based
on the max size of inspected instance vars in user classes) it
should not use too much memory.

This now allows us to support reified instance variable fields up
to an arbitrary size.

Note that although the allocators are generated on a per-class
basis (necessary without using MethodHandle for allocation) the
variable accessors are still hand-written and only call the
appropriate getVariable# or setVariable# up to 9 as before. It is
an open TODO to generate these accessors on a per-class basis so
they inline all the way through. The alternative would be moving
toward more MethodHandle-based factories, but that limits
cross-JVM support and may result in startup impact.

For #4643 among others.