The goal is to allow for more evaluation at parse time, in general, to
increase eval time performance.
- Make AST_Node non-constructable except by derived classes.
- Make data in AST_Node const (as much as possible).
- Replace reflection "text = " with replace_child() (where the
replacement must be with a new parse tree).
- Evaluate floats, strings, ints, chars at parse time to avoid repeat
evaluations (~10% speed up in loops in -O3)
