Allow strong typing of the schema and make it instance independent
Currently, the schema type definition is loosely typed and each part of the schema (etype/attribute/relation schema) contains several eids. I think that we should "standardize" the way the schema is represented in ts/js. This single representation should be supported by the following tools:
- cwclientlibjs
- cubicweb-tsgen
- rqlcontroller schema's holder
- cwelements
- rqlbrowser
- ra-cubicweb
I think that the Schema type definition and its associated utils should be located in cwclientlibjs.