Add support for mut bindings in function arguments

I noticed that this works:

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

but this doesn't:

```rust
fn check_int_slices(v: HashSet, mut lens: Vec) -> bool {
...
}
```

Error:

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