node.h: extract NODE functions from gc.c

* node.h (rb_gc_free_node, rb_node_memsize, rb_gc_mark_node):
  extract functions for NODE from obj_free(), obj_memsize_of(),
  gc_mark_children() in gc.c.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47197 b2dd03c8-39d4-4d8f-98ff-823fe69b080e