Propsoal: New API to create a writer with unknown root about js-car

[rvagg]

So this would just buffer all the Uint8Array chunks in memory until it has the roots it needs? Seems like a foot-gun prone interface so it would need some solid documentation around it - DO NOT USE THIS IF YOU HAVE A LOT OF DATA. Maybe the interface could be more suggestive too: createBufferingUntilRootsAdded() or something explicit so the user has to type out their intention before they opt in to this.

It wouldn't really even need a byteLength argument because it's going to be holding on to a list of Uint8Arrays anyway and it just needs to create the header when it gets the roots and spit that out first. Nothing about the bytes that it buffers is impacted by the length of the header. You just get a one-shot addRoots() call to open the flood gates.

But otherwise, this seems reasonable.

[Gozala]

Closing this in favor of #69