Function preprocess 

pub fn preprocess<'s>(
    src: impl Iterator<Item = SpannedToken<'s>>,
    state: &mut PreprocessorState<'s>,
    cache: &'s PreprocessorCache<'s>,
) -> Result<Vec<SpannedToken<'s>>, ()>

Preprocess the given token stream, elaborating any compiler directives

state is augmented during preprocessing (and can be examined afterwards, likely to inspect any errors found), and cache is used to retain any new files/spans found during preprocessing

preprocess returns the elaborated stream, as well as whether the initial stream was consumed completely (false if an irrecoverable error was encountered)

let file_contents = "
`define TEST(a, b) a + b
`TEST(1, 2)
";
let tokens = lex(file_contents, "test_file.v").tokens();
let mut pp_tokens = preprocess(tokens, &mut state, &cache).unwrap().into_iter();
assert_eq!(pp_tokens.next().unwrap().0, Token::UnsignedNumber("1"));
assert_eq!(pp_tokens.next().unwrap().0, Token::Plus);
assert_eq!(pp_tokens.next().unwrap().0, Token::UnsignedNumber("2"));
assert_eq!(pp_tokens.next(), None)