allow more stuff through ports

[evancz]

I am collecting various requests to allow more things through ports:

• allow Uint8ClampedArray to cross ports #1436 - Uint8ClampedArray
• https://github.com/elm-lang/core/issues/476 - undefined as null
• https://github.com/elm-lang/core/issues/899 - NaN as null (or better error message!)

Always prefer opening new issues rather than adding on to existing ones. So if you run into some need in daily work, report it separately.