Safer casts

[terranium]

Janice Caron Wrote:

>     to!(T)(x)        // convert
>     is!(T)(x)        // is upcast or downcast possible?
>     class!(T)(x)    // upcast or downcast
>     union!(T)(x)    // reinterpret the bits
>     auto!(T)(x)        // remove constancy
>     invariant!(T)(x)    // add invariance

1) I don't think you can throw away the cast keyword, this will break all existing code.
2) how about interfaces?
3) I believe reinterpret_cast was introduced as analog of C cast, D already has cast for this purpose. I don't think D needs union!().
4) D already has cast to add/remove constancy/invariance.

My proposal was just adding safe cast for downcasts with the syntax of existing cast, nothing more.