Change varobj_dynamic::child_iter to unique_ptr

This changes varobj_dynamic::child_iter to be a unique_ptr, removing
some manual management.

gdb/ChangeLog
2020-12-11  Tom Tromey  <tom@tromey.com>

	* varobj.c (struct varobj_dynamic) <child_iter>: Now unique_ptr.
	(varobj_get_iterator): Return unique_ptr.
	(update_dynamic_varobj_children, install_visualizer)
	(varobj::~varobj): Update.
	* python/python-internal.h (py_varobj_get_iterator): Return
	unique_ptr.
	* python/py-varobj.c (py_varobj_get_iterator): Return unique_ptr.