Use PRINT-SYMBOL-WITH-PREFIX to print class names.

It can be very confusing when the same symbol names are used in two different packages, and when it leads to some confusion then the error messages also don't contain package information.

This patch uses ```print-symbol-with-prefix``` also for printing class names in addition to the other usage already in the sources, making things a bit more consistent.

```
$ rgrep -i print-symbol-with-prefix .
./code/class.lisp:                    ~/sb-impl::print-symbol-with-prefix/" name)
./code/early-format.lisp:(defun print-symbol-with-prefix (stream symbol &optional colon at)
./code/condition.lisp:                     ~/sb-impl::print-symbol-with-prefix/.~:@>"
./code/condition.lisp:             (format stream "redefining ~/sb-impl::print-symbol-with-prefix/ ~
./code/condition.lisp:             (format stream "redefining ~/sb-impl::print-symbol-with-prefix/ ~
./code/condition.lisp:             (format stream "redefining ~/sb-impl::print-symbol-with-prefix/ ~
./code/target-package.lisp:                ~{~/sb-impl::print-symbol-with-prefix/~^, ~}~:@>"
./code/target-package.lisp:                              ~/sb-impl::print-symbol-with-prefix/~}~@:_~}~
./code/ntrace.lisp:                   (warn "~/sb-impl::print-symbol-with-prefix/ is ~
./code/defstruct.lisp:      ;;x  (style-warn "redefining ~/sb-impl::print-symbol-with-prefix/ ~
./pcl/std-class.lisp:                  for class ~S:~4I~@:_~<~@{~/sb-impl::print-symbol-with-prefix/~^~:@_~}~:>~@:>"
./pcl/std-class.lisp:                   ~/sb-impl::print-symbol-with-prefix/ of an instance ~
./pcl/std-class.lisp:                                  ~/sb-impl::print-symbol-with-prefix/ ~
./pcl/slots.lisp:                 (format stream "~@<The slot ~/sb-ext:print-symbol-with-prefix/ ~
./pcl/slots.lisp:                         ~/sb-ext:print-symbol-with-prefix/ ~
./pcl/slots.lisp:                         ~/sb-ext:print-symbol-with-prefix/.~@:>"
./pcl/print-object.lisp:          (format stream "~/sb-impl::print-symbol-with-prefix/ ~{~S ~}~:S"
./pcl/print-object.lisp:          (format stream "~/sb-impl::print-symbol-with-prefix/, slot:~S, ~:S"
./pcl/print-object.lisp:             (format stream "~/sb-impl::print-symbol-with-prefix/~:[~:; ~:S~]"
./pcl/print-object.lisp:          (print-symbol-with-prefix stream name)))
./pcl/cpl.lisp:              (format nil "named ~/sb-impl::print-symbol-with-prefix/"
./pcl/cpl.lisp:               (format nil "named ~/sb-impl::print-symbol-with-prefix/"
./pcl/cpl.lisp:                 (format nil "named ~/sb-impl::print-symbol-with-prefix/"
./pcl/macros.lisp:                     ~/sb-impl::print-symbol-with-prefix/." symbol))
./compiler/early-c.lisp:                 ~/sb-impl::print-symbol-with-prefix/, not the~@
./compiler/ir1report.lisp:                "~@<~@(~D~) call~:P to ~/sb-impl:print-symbol-with-prefix/ ~2:*~[~;was~:;were~] ~
./compiler/ir1report.lisp:         "~@<Proclaiming ~/sb-impl:print-symbol-with-prefix/ to be INLINE, but ~D call~:P to it ~
./compiler/ir1report.lisp:           "~@<Call to ~/sb-impl:print-symbol-with-prefix/ could not be inlined because no definition ~
./compiler/ir1report.lisp:           "~@<Call to ~/sb-impl:print-symbol-with-prefix/ could not be inlined because its source code ~
```