Support print-circle and related features.

Support print-circle and related features.
(Vprint_gensym_alist): Removed.
(Vprint_circle, Vprint_continuous_numbering, print_number_index
Vprint_number_table): New variables.
(PRINT_NUMBER_OBJECT, PRINT_NUMBER_STATUS): New macros.
(PRINTPREPARE, PRINTFINISH): Don't set Vprint_gensym_alist.
(print, print_preprocess, print_preprocess_string, print_object):
New/modified functions with print-circle feature.  Use
Vprint_number_table instead of Vprint_gensym_alist for print-gensym.
(syms_of_print): Defined new Lisp variables `print-circle',
`print-continuous-numbering', `print-number-table'.