src/{c-types-impl,method-{proto,impl}}.lisp: Improve `merge-keyword-lists'.

src/{c-types-impl,method-{proto,impl}}.lisp: Improve `merge-keyword-lists'.

  * Make the overall description of the merging site be a function (a)
    to report a file location and (b) to avoid formatting the thing if
    there isn't actually a problem.

  * Make the per-argument-list descriptions be functions so that they
    can report more than one message each.

  * Change `method-keyword-argument-lists' to report the inheritance
    path for methods involved in keyword argument conflicts.  Add an
    argument to thread through an `inheritance-path-reporter-state'
    object to make this work.