Retype traverse_interval arg type from Lisp_Object to void *

Retype traverse_interval arg type from Lisp_Object to void *

This is a refactoring.  It eliminates a few unnecessary conses and
allows for further improvements.

* src/intervals.c (traverse_intervals):
Change argument type.  All callers adapted.
* src/fns.c (collect_interval, Fsxhash_equal_including_properties)
(Fobject_intervals):
* src/print.c (print_check_string_charset_prop)
(print_prune_string_charset, print_object, print_interval):
Pass a pointer to a Lisp_Object instead of a Lisp_Object.