Roadmap for `cfg` attributes

[cdstanford]

Current infrastructure for skipping cfg attributes is hacky, see this code in scanner.rs:

    // Quickfix to decide when to skip a CFG attribute
    // TODO: we need to use rust-analyzer or similar to more robustly parse attributes
    pub fn skip_cfg(&self, args: &str) -> bool {
        args.starts_with("target_os = \"linux\"") || args.starts_with("not (feature =")
    }

    // Return true if the attributes imply the code should be skipped
    pub fn skip_attr(&self, attr: &'a syn::Attribute) -> bool {
        let path = attr.path();
        // if path.is_ident("cfg_args") || path.is_ident("cfg") {
        if path.is_ident("cfg") {
            let syn::Meta::List(l) = &attr.meta else { return false };
            let args = &l.tokens;
            if self.skip_cfg(args.to_string().as_str()) {
                info!("Skipping cfg attribute: {}", args);
                return true;
            } else {
                debug!("Scanning cfg attribute: {}", args);
                return false;
            }
        }
        false
    }

    // Return true if the attributes imply the code should be skipped
    pub fn skip_attrs(&self, attrs: &'a [syn::Attribute]) -> bool {
        attrs.iter().any(|x| self.skip_attr(x))
    }

Probably eventually we want to add code to the rust-analyzer wrapper to get information about attributes that are currently enabled for the current build. Relates to the story for build-time effects #13 as well.