remove namespace and key parameters from vnode signature

[matthieubulte]

just a little cosmetic change to extract the namespace and the key of a vnode from the properties parameter instead of using two Maybe String.

It just makes the code look better and makes it lighter to use as most of the time these two parameters are not used.

[garyb]

Much preferrable, thanks. 👍 This is the approach virtual-dom/h takes too, which if I'd know about at the time I'd probably have gone with also.