Detect 8-bit alligned type TXY by TX,TY.

[Vitaliy Fadeev]

Detect 8-bit alligned type TXY by TX,TY.

```d
     union
     {
         struct
         {
             TX x;
             TY y;
         }
         TXY xy;
     }

     alias TXY = Detect!(TX,TY);
```

Wanted:

```d
Detect( uint, uint )     == ulong;
Detect( uint, ubyte )    == ulong;
Detect( ushort, ushort ) == uint;
Detect( ushort, ubyte )  == uint;
```

Is there a D-trait or D-function for detect TXY (named for 
example ```Detect``` in code above) ?