librustc: Remove the fallback to int from typechecking.

This breaks a fair amount of code. The typical patterns are:

* `for _ in range(0, 10)`: change to `for _ in range(0u, 10)`;

* `println!("{}", 3)`: change to `println!("{}", 3i)`;

* `[1, 2, 3].len()`: change to `[1i, 2, 3].len()`.

RFC #30. Closes #6023.

[breaking-change]

src/test/run-pass/trait-inheritance-num3.rs

@@ -16,7 +16,7 @@ pub trait NumExt: PartialEq + PartialOrd + Num + NumCast {}
 impl NumExt for f32 {}
 
 fn num_eq_one<T:NumExt>(n: T) {
-    println!("{}", n == NumCast::from(1).unwrap())
+    println!("{}", n == NumCast::from(1i).unwrap())
 }
 
 pub fn main() {