operator overloading outside the type

[Steven Schveighoffer]

I think we should enable this. It was identified pretty recently 
in a stream of an influencer (see other messages in this group), 
as something that is desirable.

Especially with importC, where C does not support operator 
overloading, it should just work. Indeed, in my [raylib 
binding](https://github.com/schveiguy/raylib-d/blob/1cc76b8be3da3c1a34c54528678a62ce237c969e/source/raylib/raylib_types.d#L7), we purposely implement our own copies of all the basic POD types because we want to add operator overloads. This becomes MUCH simpler if we could just use UFCS operators.

A long time ago, member functions were member functions only, and 
UFCS did not exist (except for arrays). Now that UFCS does exist, 
and operator overloads are a straightforward template rewrite, 
the position of only allowing operator overloads to be members is 
less defendable.

I also agree with the streamer that .h files should just work.

-Steve