WIP: Return content range data immediately

[hannahhoward]

Goals

Still working on this but this would return the data needed to set a content-range header in the service worker needed for video. The only problem: big signature change to fetchContent. It now returns a promise which contains the information needed for content length AND the async iterable.

Done

• car functions changes (still messing around with these)
• Saturn.fetchContent

TO DO

• Implement fetchContentWithFallback so that it ALSO returns the same signature (not easy - for now I've just compressed the call to fetchContent)
• Implement fetchContentBuffer so that it returns a comparable signature
• Get tests passing

For discussion

The complexity add to do this is non-trivial (especially with fetchContentWithFallback). Maybe there's a better solution? I'm not sure there is though.