- FIX: `Label & const` was not actually doing the `& const` masking
(fixes#1446)
- ADD: `LOW(Label)` can be constant if `Label` is aligned to 8 or more bits
(resolves#1444)
- ADD: `!expr` can be constant 0 if `expr` has any non-zero bits
(resolves#1447)
- `LOW()` and `HIGH()` have their own RPN operator values
(resolves#1445)
The change to RPN values means that the object file version was incremented.
This also refactors unary operators and functions, combining their
evaluation similarly to binary ones.
