Added condition class reference-condition in src/conditions.lisp

* src/types.lisp: new file; contains types used by the more-conditions
  systems; currently `reference-spec' with accessors
  `reference-document', `reference-part' and `reference-link'
* src/variables.lisp: new file; contains variables used by the
  more-conditions system; current *print-references* which controls
  whether condition reports should print references
* src/conditions.lisp
  (print-reference): new function; print reference specification onto
  stream
  (condition-references): new generic function; return list of
  references associated to a condition
  (reference-condition): new condition class; mixin class for
  associating references to conditions
  (print-object :after reference-condition t): new method; print
  references associated to condition if appropriate
* src/package.lisp (package more-conditions): added exported symbols
  reference-spec, reference-document, reference-part, reference-link,
  print-reference, *print-references*, condition-references,
  reference-condition; extended documentation string
* test/conditions.lisp (reference-error): new condition class; helper
  for testing `reference-condition'
  (define-condition-suite reference-error): new test suite; for
  `reference-condition'
  (foo-error): new condition class; helper for testing
  `reference-condition'
  (define-condition-suite foo-error): new test suite; for
  `reference-condition'
* more-conditions.asd (+version-minor+): changed 1 -> 2
  (system more-conditions): added files src/types.lisp and
  src/variables.lisp