Warn or error on platform-specific integer conversions

Code like

```
let i : i64 = 0;
```

will fail to compile on 32-bit machines, because the type `int` is 32-bits, but succeed on 64-bit builds because there `int` and `i64` are the same. It'd be nice if the compiler could report cases where we're assuming particular sizes for `int` or `uint` so we don't get bit by cross-platform build failures later.
