Add more declarations to readtable manipulation.

This makes non-unicode builds elide all the GETHASH calls.
Small change to how DEFSTRUCT parses the :PRINT-blah options.

- The error message shown can generally avoid mentioning anything
  about the :TYPE option unless the :TYPE option was actually present.
- A sanity-check meant to assert (THE (OR SYMBOL CONS) (CAR ARGS))
  but it mistakenly asserted on ARGS which is tautologically nil or a cons.