Make null an explicit type (remove nullable types)

[annevk]

In #827 the idea came up that now that we have unions we could handle null in a generalized way. That is, rather than DOMString?, you would write (DOMString or null).

I like that this adds a lot of clarity with regards to the allowed values, but I do worry a bit about the verbosity. We might want to add a typedef for NullableDOMString or some such.

[domenic]

I prefer having a shorthand. This is so common, both on the web platform and in programming languages in general, that removing the shorthand is not helpful.

We could define the shorthand as being equivalent to such a union, but even then, I'd prefer we advise people to never explicitly use the union, for consistency.

[annevk]

I guess in that case you'd also be in favor of continuing to ignore null for various extended attributes as a special case? Let's close this then.

[domenic]

Right, although I'd phrase it as an operator precedence issue: [AllowShared] ArrayBuffer? is interpreted as ([AllowShared] ArrayBuffer)?, not [AllowShared] (ArrayBuffer?).