Moved some parts into mixins in src/mixins.lisp
* src/mixins.lisp: new file; mixin classes
  internal-combination-mixin, internal-handlers-mixin,
  internal-documentation-mixin and simple-printing-mixin
* src/object-internal.lisp (object-hook): added superclasses
  internal-combination and simple-printing-mixin
  (object-hook::combination): removed slot; providing mixin class
  (print-object object-hook t): removed; providing mixin class 
* src/object-external.lisp (external-hook): added superclasses
  internal-combination-mixin, internal-handlers-mixin,
  internal-documentation-mixin and simple-printing-mixin
  (external-hook::combination): removed; provided by mixin class
  (external-hook::handlers): likewise
  (external-hook::documentation): likewise
  (documentation external-hook t): remove; provided by mixin class
  (setf documentation string external-hook t): likewise
  (print-object external-hook t): likewise 
* cl-hooks.asd (system cl-hooks): added file src/mixins.lisp