cl-print.el: Reduce code duplication

cl-print.el: Reduce code duplication

While at it, fix a bug in `cl-print-object-contents` for strings,
where we forgot to pass `stream` to `princ` at one place and simplify
a `substring` call using a negative offset.

* lisp/emacs-lisp/cl-print.el (cl-print--cons-tail)
(cl-print--vector-contents, cl-print--struct-contents)
(cl-print--string-props): New functions, extracted from
`cl-print-object-contents`.
(cl-print-object, cl-print-object-contents): Use them.