Remove unnecessary comma.
Convert bool and null into atoms.

There should never be more than a single value of these types in the
same execution run of a virtual machine, as they are all conceptually
identical values.

It may be worth it to think about now a native layer can screw this up
(writing to the <i> value in a bool, for instance).
Add make_constant function.
Add dedicated constant load instruction.
Reorganize virtual machine switch statement.
Coding style.
Add conditional in test.ch for regressions.