change 'this' pointer for structs to reference type

[Janderson]

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).

I never understood why struct's in D worked this way.  When I initially 
went to use D structs I assumed 'this' would behave in the same way as 
classes.  Making 'this' a reference type for struct's makes sense to me. 
  Of course it does mean another code breaking feature like you said.

-Joel