Make it possible to define a "stack fill" value

[c3d]

trafficstars

In order to be able to more easily port old RPN programs, it might be nice to be able to define a fill value to fill the stack.

An alternative is to have a setting enforcing a fixed-size stack (with last-item filling like on RPN).

[c3d]

Part of the problem doing that is how to implement that without slowing down normal execution. A fixed size stack requires at least a modulo operation adn pre-allocation of the stack, or a test and possibly a memory move when pushing stuff. This is not free.