Delete unnecessary allocation in empty!(::Dict)

I don't know the internal `Dict` code well, but AFAICT these allocations are completely unnecessary---if `slot[i]` is 0, it doesn't seem to matter what's in `keys[i]` or `vals[i]`.

I have an application where these accounted for 90% of the runtime, hence my interest in eliminating them. If desired, I can backport to 0.3.