Add support for mut bindings in function arguments

I noticed that this works:

fn check_int_slices(v: HashSet<u64>, lens: Vec<usize>) -> bool {
    let mut lens = lens;
    ...
}

but this doesn't:

fn check_int_slices(v: HashSet<u64>, mut lens: Vec<usize>) -> bool {
    ...
}

Error:

error: no rules expected the token `lens`
   --> src/lib.rs:797:50
    |
797 |         fn check_int_slices(v: HashSet<u64>, mut lens: Vec<usize>) -> bool {
    |                                                  ^^^^ no rules expected this token in macro call