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/rgbds.5

@@ -1,6 +1,6 @@
 .\" SPDX-License-Identifier: MIT
 .\"
-.Dd December 22, 2023
+.Dd August 6, 2024
 .Dt RGBDS 5
 .Os
 .Sh NAME
@@ -391,6 +391,10 @@ The value is then ORed with $C7
 .It Li $80 Ta Integer literal; followed by the
 .Cm LONG
 integer.
+.It Li $70 Ta Ql HIGH
+byte.
+.It Li $71 Ta Ql LOW
+byte.
 .It Li $81 Ta A symbol's value; followed by the symbol's
 .Cm LONG
 ID.