
For version 2.1
1. Implement HIDE/IN/END
2. Finish help files for the builtins

For version 2.5
1. Clean up the classes to actually hide the mechanics of reference/object
from the methods called from builtin.cc.
There should not be any ->to_agg()[1] or ->to_int, I should be able to put
data on stack with simple S.push(data). Needs some thought.
2. Clean up and structurize builtins.

For version 3.0
1. Redesign the object references and object pools. Consider multi-referencing
only aggregate objects, while primitive types should be stored on stack and
manipulated as is.
