librustc: Remove the fallback to int for integers and f64 for

floating point numbers for real.

This will break code that looks like:

    let mut x = 0;
    while ... {
        x += 1;
    }
    println!("{}", x);

Change that code to:

    let mut x = 0i;
    while ... {
        x += 1;
    }
    println!("{}", x);

Closes #15201.

[breaking-change]

src/libserialize/json.rs

@@ -3424,7 +3424,7 @@ mod tests {
         hash_map.insert("a".to_string(), 1i);
         hash_map.insert("b".to_string(), 2);
         assert_eq!(hash_map.to_json(), object);
-        assert_eq!(Some(15i).to_json(), Number(15 as f64));
+        assert_eq!(Some(15i).to_json(), Number(15f64));
         assert_eq!(None::<int>.to_json(), Null);
     }