Refactor to increase modularity

[labra]

Given that ShEx and SHACL have been stable in the last years, it may make more sense to modularize this library and to divide it in at least the following libraries (repositories):

• shaclex: which will contain the converters between ShEx and SHACL and will still be the main entrance point for RDFShape
• shexcala: which will contain the shex validator
• shacls: which will contain the shacl validator
• rdfs: which will contain the simple RDF interface used by both ShEx and SHACL validators

In this way, each library/repository will evolve and be more modular. Those modules were originally separated by we decided to join them together to facilitate coding.