Adds Readable stream support when saving

[seanaye]

This PR allows passing a readable stream to the fileSave function instead of a blob. When supported this will pipe bytes directly into a writable stream, which reduces memory usage on large files (prevents copying). When not supported, the stream will simply be converted to a blob

closes #66

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[tomayac]

I think this is headed in a great direction. Just one remark regarding the wrapping object. (Ignore the other comment that I have clarified myself.)

[tomayac]

Some additional comment suggestions.

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[seanaye]

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[tomayac]

OK, here's the way this could work:

You pass it either a Blob, or a Response, which can contain a ReadableStream and has a type attribute:

const response = new Response(new ReadableStream(), {
  headers: {
    "content-type": "text/plain",
  },
});

This means we don't need the weird object! I have made suggestions to implement this below, but I probably missed stuff (did it on the front-end without actually testing), so please carefully check this again.

[tomayac]

@seanaye Are you OK with the solution I sketched in #67 (review)?

[seanaye]

Yea definitely, sorry haven't had the time to make the changes

[tomayac]

LGTM