Cleaner approach to things related to stores and queries

* got rid of the Free[Command]
* RDFStore is now a typeclass
* it is made of 3 three independent ones: GraphStore, SparqlEngine, Transactor
* there is an additional Lifecycle typeclass for start/stop operations
  * it comes with a default instance doing nothing
* syntax enhancement are made available from within the typeclass themselves
This PR is not about fixing all the typeclass instances. It is about cleaning up the typeclass definition.

That being said, I have just implemented the change in https://github.com/w3c/banana-rdf/commit/23ed8886345d582c1ca0f49237e1f3823e0c4543 .