`std::visit` is (arguably) cleaner code, but older versions of gcc
and clang (not very old; the ones packaged with Ubuntu 22.04 LTS)
compile them as tables of function pointers, instead of efficient
jump tables.
* Fixes from temporarily re-enabling more compiler warnings
* More edits suggested by cppcheck
* Fix hanging on append_yylval_string
* Fix FOR loop increment
Allows better platform-agnostic path manipulation.
Also, using `std::optional` rather than empty strings allows
correctly handling empty arguments (treating them as such,
instead of acting as they were never passed).
