Basic logic for parameterless %defines. TODO: %define with parameters,
`_apply_defines()`. Later: %include, %error, %macro and %endmacro.
That will probably be all at the preprocessor level for now; I've got
just little over a month to make something usable out of this.
Let's say a week for parser, week for generator (hopefully :D),
week for adding ELF (object files only? probably invoking `ld` if
the file should be linked) and a week for bugfixing.