Entity sets and engine api through systems

* src/system.scm: entities and components in a cons-pair of two hash
                  tables and systems returning entities changes
                  directly

* src/engine.scm: modifying an engine is possible only using systems
                  and each system modify engine entities when runs

* src/examples/entity-componentfunctions.scm,
  src/examples/making-systems.scm,
  src/examples/composing-systems.scm,
  src/examples/engine-conway-game.scm:
        modifications for correct execution with the new state of art