* print.c (scm_iprin1): Enter printed structures into the print

* print.c (scm_iprin1): Enter printed structures into the print
state as nested data while they are printed.
(print_state_fluid, print_state_fluid_num): New variables.
(scm_init_print): Initialize them.
(scm_iprin): If print_state_fluid carries a print_state, use that
instead of creating a new one.
(scm_printer_apply, apply_stub, struct apply_data): New
definitions to help with calling printer functions written in
Scheme.
* print.h (scm_printer_apply): New prototype.