Review of std.signal

[Kagamin]

1. there're two typical usage patterns of signals: you mostly 
have 0 or 1 slots attached, so it would be nice if having one 
slot wouldn't require allocation. SlotsArray should be probably a 
tagged union of SlotImpl and SlotImpl[]. Is it worth an effort?
2. why InvisibleAddress is so big? Isn't it enough to just negate 
it?
3. I think, it's important to document, that a weak connection 
can be destroyed before it becomes logically unneeded. It's an 
important and non-trivial caveat.