The preprocess_* functions are now privately contained within the
implementation file to help the preprocesser outer function.
Furthermore I've simplified the API of the preprocess_* functions by
making them only return pp_err_t and store their results in a vector
parameter taken by reference.
We leave the parameter tokens alone, considering it constant, while
the parameter vec_out is used to hold the new stream of tokens. This
allows the caller to have a before and after view on the token stream
and reduces the worry of double frees.
This C++ rewrite allows me to rewrite the actual API of the system.
In particular, I'm no longer restricting myself to just using enums
then figuring out a way to get proper error logging later down the
line (through tracking tokens in the buffer internally, for example).
Instead I can now design error structures which hold references to the
token they occurred on as well as possible lexical errors (if they're
a FILE_LEXICAL_ERROR which occurs due to the ~%USE~ macro). This
means it's a lot easier to write error logging now at the top level.
I've decided to split the module parsing into two modules, one for the
preprocessing stage which only deals with tokens and the parsing stage
which generates bytecode.
