Typed FlxSignal

Added type parameters to FlxSignal.
It is a rip-off version of msignal with only the essential components remain.

I didn't implement the persist property because I believe it should be the user to handle this. They can make use of the stateSwitch signal in SIgnalFrontEnd.

And no more pooling due to the complexity of implementing a pool on typed signals.

Please feel free to discuss.