Make expression evaluation lazy

[ISSOtm]

Closes #663.

TODO:

• [x] Make constant expression evaluation not produce error messages, but only RPN error bytes
• [ ] Adapt the rest of the code
• [x] Write rpn_Reduce, which attempts to compute an expression's value, and actually produces the error messages if not
• [ ] Bonus: reuse rpn_Reduce in RGBLINK?

[ISSOtm]

This is actually hell to implement. What I've done so far lays some groundwork, but it's lacking.

The most difficult part is actually being able to defer error reporting! This is much, much more involved than it seems.