Suggestions for toplevel_ref_arg should not move mut to the right-hand side

[joshtriplett]

When clippy sees code like this:

let ref mut foo = false;

It'll suggest changing it to this:

let foo = &mut false;

However, while ref on the left-hand side is not idiomatic Rust, mut on the left-hand side is, and often the code would look much clearer if written as:

let mut foo = false;

(With corresponding changes to code using foo.)