More rigorous type checking?

[mbostock]

We should consider checking for some invalid types earlier, if possible, rather than deferring errors until the transition starts.

• [x] transition.attrTween’s value should be a function that returns a function (interpolator)
• [x] transition.styleTween’s value should be a function that returns a function (interpolator)
• [x] transtion.tween’s value should be a function that returns a function
• [x] transition.on’s listener should be a function
• [x] transition.ease’s value should be a function