Optimise clearing and traversing code to only re-parse the modified file

[JoshuaBatty]

When traversing, skip over AST nodes that originate from cached modules and only traverse and collect tokens from the file that was modified.

TokenMap

/// Remove all tokens for the given file from the token map.
pub fn remove_tokens_for_file(&self, path_to_remove: &PathBuf) {
    self.0.retain(|key, _value| {
        key.path.as_ref().map_or(true, |path| path != path_to_remove)
    });

the in the traverse function

pub fn traverse(
    results: Vec<(Option<Programs>, Handler)>,
    engines_clone: &Engines,
    session: Arc<Session>,
    file_versions: &BTreeMap<PathBuf, Option<u64>>
) -> Result<Option<CompileResults>, LanguageServerError> {
    // Clear the tokens out of the TokenMap if the file has changed.
    let modified_file = file_versions.iter().find_map(|(path, version)| {
        version.map(|_| path)
    });
    match modified_file {
        Some(path) => {
            session.token_map.remove_tokens_for_file(path);
        },
        None => {
            session.token_map.clear();
        },
    }
}