Support unimplemented interface implementations in src/api/classes.lisp

* src/api/types.lisp: new file; contains types used by the frontend
  module
* src/api/conditions.lisp: new file; contains conditions used by the
  frontend module
* src/api/classes.lisp (header): updated copyright
  (define-interface-implementations): when encountering an unmapped
  interface implementation during deserialization, insert a marker
  object and signal a condition; deal with marker objects during
  serialization
* jenkins.api.asd (system jenkins.api): added files src/api/types.lisp
  and src/api/conditions.lisp