Change both records and named structures to store default values as

Change both records and named structures to store default values as
thunks in the type structure, which are then called when needed.

Introduce new procedures to get the default value for a slot, given
the type descriptor, and use them as needed in DEFINE-STRUCTURE,
rather than just inserting the default-init expression.

Put back the UNPARSER-METHOD argument to MAKE-RECORD-TYPE, and use it
in DEFINE-STRUCTURE.

Once again, use RECORD-KEYWORD-CONSTRUCTOR in DEFINE-STRUCTURE, this
time with better results.