Implement match ergonomics (binding modes) in type inference

[flodiebold]

E.g.

let x = Some(1);
match &x {
    Some(n) => n, // n should be &i32
}
match &mut x {
    Some(n) => n, // n should be &mut i32
}

and so on. We currently just infer wrong types in this situation. To fix this, when walking through patterns with infer_pat, we need to keep track of the 'binding mode' and apply it when creating bindings. This is also explained in the rust reference; and I think rustc handles it here.