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]
The `~str` type is not long for this world as it will be superseded by the
soon-to-come DST changes for the language. The new type will be
`~Str`, and matching over the allocation will no longer be supported.
Matching on `&str` will continue to work, in both a pre and post DST world.
