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/libnative/io/c_unix.rs

@@ -93,7 +93,7 @@ mod select {
     }
 
     pub fn fd_set(set: &mut fd_set, fd: i32) {
-        set.fds_bits[(fd / 32) as uint] |= 1 << (fd % 32);
+        set.fds_bits[(fd / 32) as uint] |= 1 << ((fd % 32) as uint);
     }
 }