change 'this' pointer for structs to reference type

[Andrei Alexandrescu]

Robert Fraser wrote:
> 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.

Unfortunately that won't be possible because references can't be null.

Andrei