1.0.20.25: Produce a loadable FASL when compiling an incompatible DEFSTRUCT.

1.0.20.25: Produce a loadable FASL when compiling an incompatible DEFSTRUCT.

* Add some tests to see that redefining a STRUCTURE-CLASS works as one
  might expect, and that compiling a file whose loading redefines a
  STRUCTURE-CLASS works, too.

* There are some nasty intermediate states having to do with
  subclasses defined in separate files (some are described in the
  tests), but that's not actually new.