Warn on shadowing built-in types

I got a little confused by the following code:

``` rust
struct Foo<T> {
    value: T
}

impl<u32> Foo<u32> {
    fn impl_func(&self) -> u32 {
        42
    }
}
```

rustc gives the error: `42: expected type parameter, found integral variable`.

The mistake is that I've created a type parameter named `u32`. Could we warn on this case? I can't see any legitimate use case for shadowing `u32`.
