pin

WARNING: No migration performed! That needs to come in a separate
commit, perhaps amended into this one.

This is the minimal rewrite, only changing the storage from
JSON(+extra keys) in Datastore to IPFS objects. All of the pinning
state is still loaded in memory, and written from scratch on Flush. To
do more would require API changes, e.g. adding error returns.

Set/Multiset is not cleanly separated into a library, yet, as it's API
is expected to change radically.

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>