D2 weak references

[Sean Kelly]

Jason House wrote:
> Jason House wrote:
> 
>> Tango's GC (and therefore
>> druntime's GC) has an explicit notification mechanism.
> 
> I guess I shouldn't have assumed that the features of tango became part of 
> druntime.  I don't see any notification mechanism :(

Same as Tango:

alias void delegate(Object) DEvent;
extern (C) void rt_attachDisposeEvent(Object h, DEvent e);
extern (C) void rt_detachDisposeEvent(Object h, DEvent e);

I should probably expose these in core.runtime.