Re: [fetch] Introduce convenience property for request's context (#93)

So I'm convinced that `document` is better than a bunch of values that are actually initiators of sort. And that "type" is a better name than "context". However, I still think we want to be specific where we can with types, for reasons @mikewest pointed out.

For `XMLHttpRequest`, `fetch()`, `rel="prefetch"`, etc. we should probably have an `any` type.

I think the different workers each deserve their own type since they represent different global objects, but I might be able to be persuaded. I think combined with my previous comment this gives me enough to go on to specify a new model tomorrow.

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