* Use clang-tidy `misc-include-cleaner` for IWYU `#include` cleanup
* Use `std::optional<size_t>` instead of `ssize_t`
* Rename some functions in linkdefs.hpp
* Fix header order
This feature is referred to as "code/data literals" in ASMotor,
and simply as "literals" in some older assemblers like MIDAS
for the PDP-10. RGBASM already had the "section fragments"
feature for keeping disparate contents together when linked,
so these worked naturally as "fragment literals".
Part of that condition's purpose is to ensure that we read the correct
lexer state; but it's possible now for the fstack to be non-empty
*before* the lexer state is registered, i.e. if there is an error
in the function that registers it.
This causes a NULL pointer deref.
* 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
Large sizes are more efficient when it's actually buffered,
but most of the time `mmap` is used instead, and the extra size
just slows down allocation of lexer states.
This removes the last use of `strdup`
This required making a lot of related pointers be `const`.
That in turn conflicted with the need to `munmap()` a pointer
eventually, which was similar to the need to eventually `free()`
an `Expansion`'s contents, so I used the same solution of a
`union`. That lets us normally use the `const` pointer for
`const` correctness, and the non-`const` one for the not-really-
mutating destruction cases.
Split the declarations into those required for the `%union` and
those only required for the `code`.
Only declare functions on top; define them at the bottom.
