How to define opCast from native types

[Cherry]

> alias BitV48 = BitVector!48;
> ulong ull;
> BitV48 bits = cast(Bit48) ull.toBitVector;
>

I find it necessary to make the user *cast* explicitly. I just 
wanted to add that though I am comfortable with this syntax, it 
is not the most efficient way since two methods toBitVector and 
opCast are being invoked. That would not be the case if D had 
provision for opCastLeft. And the syntax would have been smoother 
too.

BTW I still have a ull.toBitVector!N, but aligned with 
std.conv:to functionality it makes a runtime check and raises 
ConvOverflowException if required.

regards
- Cherry