librustc: Remove cross borrowing from mutable `Box`es to `&mut`.

This will break code like:

    fn f(x: &mut int) {}

    let mut a = box 1i;
    f(a);

Change it to:

    fn f(x: &mut int) {}

    let mut a = box 1i;
    f(&mut *a);

RFC 33; issue #10504.

[breaking-change]
auto merge of #15171 : pcwalton/rust/remove-cross-borrowing, r=brson

This will break code like:

    fn f(x: &mut int) {}

    let mut a = box 1i;
    f(a);

Change it to:

    fn f(x: &mut int) {}

    let mut a = box 1i;
    f(&mut *a);

RFC 33; issue #10504.

[breaking-change]

r? @brson