Cannot transmute type parameters

It would be nice if there would be some way to `std::mem::transmute` in functions, even with type parameters.

Maybe the prototype of the `transmute` function could be changed to

```
fn transmute<T,U:Transmutable<T>>(x: T) -> U;
```

where `Transmutable` is a compiler-implemented type, like `Send`, `Copy` or `Share`.
