* 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
- "Double quotes" for strings (filenames, section names, CLI option arguments, etc)
- 'Single quotes' for characters and CLI option flags
- `Backticks` for keywords and identifiers (symbol names, charmap names, etc)
CLI option flags also have their leading dashes
- Single unified routine for erroring out or handling missing dependencies
- Single three-state enum instead of two Booleans for missing dependencies
(this causes `-MC` to imply `-MG` instead of needing `-MG -MC`)
- Functions than can miss included files return a Boolean for whether the
parser should `YYACCEPT` and exit
* Remove `err` and `warn`, keep `errx` and `warnx`, using them in RGBGFX too
* Separate RGBGFX and RGBLINK warnings/errors from main options
* Separate `report` function into `error` and `fatal` messages
* Implicit newlines for most RGBASM errors
Identify files by (device, inode), not by path, so that symlinks,
relative paths, case-insensitive paths, or other edge cases
do not result in double includes.
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
