r9796: * sql/expressions.lisp: reactivate caching of generated SQL

r9796:         * sql/expressions.lisp: reactivate caching of generated SQL
        strings.
        Move methods for DATABASE-OUTPUT-SQL, OUTPUT-SQL and SQL-HASH-KEY
        here from sql/fdml.lisp. Rationalise behaviour of SQL-OUTPUT,
        OUTPUT-SQL and DATABASE-OUTPUT-SQL.
        * sql/fdml.lisp: remove disabled method ADD-STORAGE-CLASS. Move
        methods for DATABASE-OUTPUT-SQL, OUTPUT-SQL and SQL-HASH-KEY to
        sql/expressions.lisp.
        * sql/ooddl.lisp: replace call to DATABASE-OUTPUT-SQL in
        DATABASE-PKEY-CONSTRAINT with call to SQL-OUTPUT.
        * sql/generics.lisp: add docstrings.