librustc: Remove cross-borrowing of Box<T> to &T from the language,

except where trait objects are involved.

Part of issue #15349, though I'm leaving it open for trait objects.
Cross borrowing for trait objects remains because it is needed until we
have DST.

This will break code like:

    fn foo(x: &int) { ... }

    let a = box 3i;
    foo(a);

Change this code to:

    fn foo(x: &int) { ... }

    let a = box 3i;
    foo(&*a);

[breaking-change]

src/libsyntax/print/pprust.rs

@@ -1862,7 +1862,7 @@ impl<'a> State<'a> {
             ast::SelfExplicit(ref typ, _) => {
                 try!(word(&mut self.s, "self"));
                 try!(self.word_space(":"));
-                try!(self.print_type(*typ));
+                try!(self.print_type(&**typ));
             }
         }
         return Ok(true);