Store pins in MFS

Motivations:

* Free HAMT. We can update the pinset without having to load/store the *entire* pinset. See #4673.
* *One* pinset. This should simplify GC.
* Allows use to deprecate (but not remove) the `ipfs pin` command while not having to maintain a bunch of deprecated code.
* Applications will *have* to use MFS (for namespaces/labels).

Blockers:

* An `ipfs files cp --prefetch` flag, maybe even a `--prefetch=background` flag?
* A way to not pin recursively in MFS. This will be tricky... We could have some form of side table but I'd rather not. Really, I'd like pin/prefecth policies in unixfs. However, this is *hard*.
* Link to IPLD from IPFS.
* Probably https://github.com/ipfs/ipld-unixfs.