change 'this' pointer for structs to reference type

[Robert Fraser]

Walter Bright wrote:
> Andrei suggested this change, and I think it is a great idea. D ought to 
> be moving away from pointers for routine work, and this would make a lot 
> of sense.
> 
> It would entail some changes to user code, mostly when dealing with the 
> opCmp and opEquals member functions. The function signatures for them 
> would be changed to opCmp(ref const S).

While we're at it, let's have "in" return a reference instead of a 
pointer for AAs.