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.

man/rgbasm.5

@@ -2,7 +2,7 @@
 .\"
 .\" SPDX-License-Identifier: MIT
 .\"
-.Dd December 22, 2023
+.Dd August 6, 2024
 .Dt RGBASM 5
 .Os
 .Sh NAME
@@ -326,6 +326,7 @@ with a non-zero constant as either operand will be constant 1, even if the other
 .Pp
 .Ic \&!
 returns 1 if the operand was 0, and 0 otherwise.
+Even a non-constant operand with any non-zero bits will return 0.
 .Ss Fixed-point expressions
 Fixed-point numbers are basically normal (32-bit) integers, which count fractions instead of whole numbers.
 They offer better precision than integers but limit the range of values.
@@ -557,6 +558,9 @@ has been defined, FALSE (0) otherwise.
 String constants are not expanded within the parentheses.
 .It Fn HIGH arg Ta Returns the top 8 bits of the operand if Ar arg No is a label or constant, or the top 8-bit register if it is a 16-bit register .
 .It Fn LOW arg Ta Returns the bottom 8 bits of the operand if Ar arg No is a label or constant, or the bottom 8-bit register if it is a 16-bit register Pq Cm AF No isn't a valid register for this function .
+The result may be constant if
+.Nm
+is able to compute it.
 .It Fn ISCONST arg Ta Returns 1 if Ar arg Ap s value is known by RGBASM (e.g. if it can be an argument to
 .Ic IF ) ,
 or 0 if only RGBLINK can compute its value.