Add #n=object, #n#, and #:symbol constructs to printer.

Add #n=object, #n#, and #:symbol constructs to printer.
(PRINTDECLARE): New macro to declare required variables.
(PRINTPREPARE, PRINTFINISH): Set printed_genyms to nil.
(Fwrite_char, write_string, write_string_1, Fterpri, Fprin1,
Fprin1_to_string, Fprinc, Fprint): Use new macro PRINTDECLARE.
(print): Print uninterned symbols readable.
(syms_of_print): Defvar `print-gensym', staticpro printed_gensyms.