`fuzz!` macro doesn't support mutable bindings

[cbiffle]

Test case resembles:

        fuzz!(|mut data: &[u8]| {
            loop {
                match hubpack::deserialize::<Structy>(data) {
                    Err(_) => break,
                    Ok((_, rest)) => data = rest,
                }
            }
        });

Alas:

28 |         fuzz!(|mut data: &[u8]| {
   |                    ^^^^ no rules expected this token in macro call

I assume the fuzz! macro is currently parsing the closure header too narrowly.