Overloading Operator !

[Chris Miller]

Overloading unary ! for classes is not a good idea because it should  
always check if the reference is null; however, I believe it should be  
overloadable for structs.

Supporting unary ! for structs can be as simple as using opEquals(bool).  
This could also allow if(mystruct).

I would currently use this feature in my dstring module to mimic D's  
if(array) (which is pretty much the same thing, a struct with fancy  
if(array) and if(!array) syntax - so why not give users this power for  
their [struct] types).