Signed and indexed packaging proposal.

Dmitry from the Chrome team has put together a packaging proposal at
https://discourse.wicg.io/t/proposal-packaging-for-the-web-signed-and-indexed/1827
that's relevant to this group's interests. Review would be ever so much
appreciated.

Brad and Yan in particular: this is somewhat along the lines of ideas
you've talked about at TPAC and elsewhere. WDYT?

Martin: This reminds me a little bit of the blind caching proposal you made
a year or three ago, at least if you squint a bit at the notion of a
resource served from one origin asserting it's provenance from another.
WDYT?

-mike