Create a helpful error message when typing "mut ref" instead of "ref mut"

[locka99]

I've gotten the order of mut and ref mixed up a few times like this:

let mut ref y = &x;

This generates a "error: expected identifier, found keyword ref" error which isn't too helpful for people used to languages that allows the order of keywords to be mixed e.g. static and const in C/C++.

The compiler should suggest "did you mean 'ref mut'?" for the above.

[GuillaumeGomez]

Taking a look.