full compilation to expr-opgen TODO:
- word! and long! are a mess right now
- I don't think word-at and long-at could use it

- Could custom forms compile to opgens?? What would this look like?
  * see far-ref - it's really a constant, there's no reason to stuff the result into the temporary register just to push it onto the stack
  * if you call expr-poly / expr-word / expr-long, then put it into the register
  * but if you call expr-opgen, just return it! (opgen.setup) returns the appropriate code if needed
  * toolbox calls could actually benefit from this! function calls, not so much
  * currently expr-opgen is assumed to not have a cleanup step - this could complicate things