Unixfsv1-directories-as-ADLs proposal

[warpfork]

Long story short: this is a proposal to renovate the way we work with unixfsv1 data so that it becomes managable through the interface of an IPLD "ADL" (an "Advanced Data Layout"), which is an API concept design to handle needs like sharding.

The aim is that this should produce nicely maintainable code, while also fixing a lot of bugs along the way, providing a couple of nice features immediately (like Selectors working over unixfsv1 pathing, which is extremely neat), and providing a solid ground for new work.

[rvagg]

This needs to highlight right up front that this is strictly about the Go implementation--it's mentioned, but a little offhand and I don't think it'll be entirely obvious to many who are reading this.

Also IMO it'd be worth extra-highlighting that one of the highest points of leverage here is the independence of UnixFS and having it all in one, well-defined place with clean boundaries, and not having its logic spilling over in many places. Being able to use it independently and have it work for all of the difficult cases it needs to work (like scalability) out of the box is going to unlock a lot of potential ecosystem & independent tooling.