Add Runtime.instance to get HINSTANCE on Windows

As promised here: https://github.com/D-Programming-Language/druntime/pull/706#issuecomment-37784396
How about using [`externDFunc`](https://github.com/D-Programming-Language/druntime/blob/f888fbcff82f30c549a7cccd98191dedcc3101ec/src/core/internal/traits.d#L57) instead?