Multiple fixes and enhancements to RPN behavior: (#1448)

- 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.

test/asm/const-and.out

@@ -1,6 +1,6 @@
 $0
 $A
-$A
+$2
 $0
 $A
 $0