* Move storage impl into `storage::cob`
* Rename `change::Create` to `change::Template`
* Rename `change::Storage::create` to `change::Storage::store`
Some of the renames are to further distinguish COBs from Changes, and
the different types of "create" operations within cobs.
Signed-off-by: Alexis Sellier <alexis@radicle.xyz>