Faster way to .dup an AA?

[Jarrett Billingsley]

I'm currently doing something like this:

int[char[]] source;

...

int[char[]] dest;

foreach(k, v; source)
    dest[k] = v;

But that loop is not very high-performance.

Is there any other way to dup an AA that's better performance?  Something is 
telling me "no."  It would be so nice if this were a supported property for 
AAs, so the implementation could come up with a faster way to .dup it..