sslot 0.3 release (is buggy atm)

[Lutger]

Sean Kelly wrote:
<snip
> 
> The tradeoff here is twofold: first, the signal mechanism must acquire 
> two locks to send a signal, and second, the processing of add/remove 
> requests is asynchronous when a signal is being sent.  Still, it only 
> uses one lock and behavior is fairly predictable.
> 
> 
> Sean

Thank you very much! I realized a similar scheme is also necessary for 
single-threaded code, since removal *may* happen during signalling if 
the GC is run.

My apologies for this bug. I actually had this solved in some previous 
implementation but it slipped back in again.