Add Runtime.instance to get HINSTANCE on Windows

As promised here: https://github.com/D-Programming-Language/druntime/pull/706#issuecomment-37784396
Then that defeats the point of this pull request - to get the `HINSTANCE` whether the code is in a .exe or a .dll.

An example use case is getting a Windows resource from the current module. The user shouldn't need to specify the instance to every call of `getResource` or whatever.