Proposal: CID+Block=Multiblock

We already have multibase, multihash that all in nutshell are metada+data. We don not however have similar thing for blocks, so it becomes impossible to derive what codec to use to decode it.

In the past when I was working on https://github.com/gozala/ipdf/ I came up with cid+block thing that I called inline blocks, so that graphs could contain encrypted and concealed sub-graphs that would only reveal themselves to the key holder.

CAR format seems to also pair CID+blocks.

And this thread https://github.com/multiformats/js-multiformats/pull/36#issuecomment-694619835 I think also illustrates lack of such abstraction.

Ironically JS Block instance also contains CID+Block but when you encoded you can no longer decode it back without additionally providing 'codec' information.

I think if we do formalize such a building block it would allow for a nice and compos-able libraries around it.

Start by reading the existing multibase and multihash abstractions, the linked CAR format specification, the JS Block reference, and the referenced js-multiformats discussion. Done means agreeing on and formalizing a CID+block abstraction that preserves enough codec information for decoding and supports composable libraries.