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.

include/asm/rpn.hpp

@@ -45,11 +45,7 @@ struct Expression {
 	void makeStartOfSection(std::string const &sectName);
 	void makeSizeOfSectionType(SectionType type);
 	void makeStartOfSectionType(SectionType type);
-	void makeHigh();
-	void makeLow();
-	void makeNeg();
-	void makeNot();
-	void makeLogicNot();
+	void makeUnaryOp(RPNCommand op, Expression &&src);
 	void makeBinaryOp(RPNCommand op, Expression &&src1, Expression const &src2);
 
 	void makeCheckHRAM();