change 'this' pointer for structs to reference type

[Walter Bright]

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).