Add signal type that only accepts one slot

[fr00b0]

If a signal only has a single slot connected, we could allow that arguments are moved into the operator() and then forwarded into the slot. This might open up some interesting use-cases, where the signal consumes the argument by taking ownership. Since we can't have several slots taking ownership of the same thing, we would need a new type of signal that only allows a single slot to be connected.

This issue was spawned from #4