By introducing the virtual "if" instruction we can clean up Boolean so
that we no longer have to refine the True and False instances. If we
also clean up Nil in a similar way it becomes easier to rewrite the
compiler in Inko, as it no longer needs to support the refining of
random objects.

This was originally introduced in
ecd11a07, but later reverted. In this
commit we re-introduce these changes. Refining True and False makes
various compiler internals in the self-hosting compiler less efficient,
as it has to support refining any object even when this only happens for
True, False, and Nil. By cleaning up True and False we can reduce this
down to just Nil, making it easier to find a way of cleaning that up
too.