Cancel waiting threads if signal disconnects all connections

[Sleitnick]

Closes #163

This also fixes a potential for FireDeferred to invoke disconnected handlers in scenarios in which a handler disconnects another handler.

[Sleitnick]

Due to potentially breaking changes, this is a major version bump.

For example:

local s = Signal.new()

task.spawn(function()
	print("Wait...")
	local res = s:Wait()
	print("Got res:", res)
end)

s:DisconnectAll()
s:Fire(32)

Previous Behavior

Before this change, DisconnectAll (and also Destroy) would not cancel out threads waiting via the Wait method. Thus, the above code would successfully print "Got res: 32" when the signal is fired, even though DisconnectAll was called.

New Behavior

This PR would prevent the "Got res: 32" output, because DisconnectAll cancels the waiting thread.

[Sleitnick]

Another behavior change is regarding FireDeferred, which will now cancel its scheduled events if the signal is disconnected before the tasks are fired.

local s = Signal.new()

s:Connect(function(n) print(n) end)

s:FireDeferred(10)
s:DisconnectAll()

Previous Behavior

The connection will still pick up the deferred event, and print 10.

New Behavior

The deferred firing will be cancelled, and nothing will be outputted.