Eliminate deprecation warning from test
Ignore some unused variables.
Make :sb-show less noisy. Only if debugging stream issues might one care.
Get plain-old DEFSTRUCT mostly working as early as possible.

Part 1:
- Avoid package-lock stuff until early-package.lisp is cross-compiled
  so that WITH-SINGLE-PACKAGE-LOCKED isn't mistaken for a function.
  (Can't just use the host's macro since that is a no-op)
- Ensure that REGISTER-LAYOUT can find STRUCTURE-OBJECT's classoid.

Part 2:
- Allow the cross-compiler to dump some subtypes of STRUCTURE-OBJECT
  that are not STRUCTURE!OBJECT.

EXPANDER-FOR-DEFSTRUCT is now a function, not a macro,
and is present in the target image to make it less magic.

Also freeze-type of defstruct-{description,slot-description}.
Don't try to call PACKAGE-IMPLEMENTS-LIST without :sb-package-locks.