* Implement custom generic tagged union `Either`
This should be more efficient than `std::variant`, while still
keeping runtime safety as it `assert`s when `get`ting values.
* Use `Either` for RPN expressions
* Use `Either` for file stack node data
* Use `Either` for `File` buffer
* Use `Either` for `STRFMT` args
* Use `Either` for RGBLINK symbol values
* Support an equivalent of `std::monostate` for `Either`
* Use `Either` for lexer tokens
* Use `Either` for symbol values
* Use `Either` for lexer mmap/buffer state
- 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.
- Since we have style rules to include foo.hpp at the top of its
corresponding foo.cpp, this takes any headers included by foo.hpp
as being also guaranteed for foo.cpp.
- Use C-style <foo.h> instead of <cfoo>, since the latter only
guarantees putting symbols in the `std` namespace, which we are
not using for C functions (e.g. `printf` not `std::printf`).
- Remove now-unused `__PRETTY_FUNCTION__` reporting
We were not initializing some expressions, and they were using
the values of the previous expressions instead. This just so
happened to not crash the tests, and to sometimes even give valid
results (although `BANK()` of a non-label symbol being $4B4E4142,
the ASCII balue of "BANK", was something we missed).
