C++20 preprocessor of C++ written in Rust

Currently, a custom build of lalrpop is needed. I'll find a proper solution to this, or replace it with antlr, which is now supported.

This is a very simple and most certainly wrong preprocessor for C++. This was not done with any major intents, it was simply to test Rust and its capabilities (I find learning by doing a lot more useful than just following tutorials).

As a rust novice, I do not claim this to be of any quality.

So far, the most rellevant missing things are:

• Module tokens are not implemented in any way. I'm waiting to getting some field experience before even looking at implementing it.
• String literal rule is way too lax. This is due to the limitations of the regex engine logos uses (Although I could revalidate the string in a following action. So, still a TODO). This does not cause a missclasification of token, so it's ok.
• the #line directive is not supported (and probably won't for some time, I do not intend to support generated code for now :) )
• QOL Features of preprocessors, like any pragma directive (none mandated by standard, but #pragma once is expected from any sensible implementation), or the __FUNCTION__ macro (which requires the step 7 parser to be implemented in order to know such information)
• Most test macros are kinda useless right now. __has_cpp_attribute is literally just hardcoded to 0

I'd say that the first 4 steps of the compilation process of C++ are done-ish! Time for lexing.

If you want more logs on what's going on, you can use the environment varaible RUST_LOG, like so: RUST_LOG=debug