Add Runtime.instance to get HINSTANCE on Windows

As promised here: https://github.com/D-Programming-Language/druntime/pull/706#issuecomment-37784396
If you use `extern(D)` you could even overload these.
```d
extern(D) HINSTANCE hInstance() @property { return _hInstance; }
extern(D) void hInstance(HINSTANCE h) @property { _hInstance = h; }
```
Later you can declare them in a different module using this.
```d
alias hInstance = externDFunc!("rt.dmain2.hInstance", HINSTANCE function() @property);
alias hInstance = externDFunc!("rt.dmain2.hInstance", void function(HINSTANCE) @property);
```