signal slots (new, fixed version)

[Lutger]

After Bastiaan Veelo's post I realized my signal slots library should be 
rewritten, which is now mostly done. You can get the result or browse 
the docs here:
lutger.ifastnet.com

Unreferenced objects are now properly garbage collected and 
disconnected, emitting a signal (should) incurs less overhead,
both memory- and performance wise.

It still needs some testing and a little work, but it's functional.

Features:
- free functions, delegate literals and delegates can act as slots
- clean syntax, freestanding signals.
- emitting signals are locked to prevent possible stack overflow, can be 
turned off with version=Unsafe.
- can set a default handler, which is invoked when a signal is emitted 
that has no slots connected.
- signals can have return values.
- signals have opApply and opApplyReverse for custom iteration, mapping 
and combining return values.

Example:

import std.stdio, sslot.signal;

class Button
{
     this() { onClick = new Signal!(); }
     Signal!() onClick;
}

class Receiver : ISlotTracking
{
     void handleClick() { writefln("button clicked"); }
     mixin SlotTracking;
}

Button   button   = new Button;
Receiver receiver = new Receiver;
button.onClick.connect(&receiver.handleClick, receiver);
button.onClick();  // prints 'button clicked'

Signal!(int,int) signal = new Signal!(int,int);

signal ~= (int num) { return num * num; };
assert(signal(3) == 9);