D operator overloading. Why that way?

[Hauleth]

Is there any reason to allow overload `op=`? IMHO it should be 
left illegal and should be interpreted always as `a = a op b`. It 
will be simpler and less confusing.

Also why `opEquals` is independent from `opCmp`? Once again 
`opEquals` should be removed and equality will be provided by 
`opCmp() == 0`.