C++-ify varobj iteration

C++-ify varobj iteration

This changes the varobj iteration code to use a C++ class rather than
a C struct with a separate "ops" structure.  The only implementation
is updated to use inheritance.  This simplifies the code quite nicely.

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

	* varobj.c (update_dynamic_varobj_children, install_visualizer)
	(varobj::~varobj): Update.
	* varobj-iter.h (struct varobj_iter): Change to interface class.
	(struct varobj_iter_ops): Remove.
	(varobj_iter_next, varobj_iter_delete): Remove.
	* python/py-varobj.c (struct py_varobj_iter): Derive from
	varobj_iter.  Add constructor, destructor.  Rename members.
	(py_varobj_iter::~py_varobj_iter): Rename from
	py_varobj_iter_dtor.
	(py_varobj_iter::next): Rename from py_varobj_iter_next.
	(py_varobj_iter_ops): Remove.
	(py_varobj_iter): Rename from py_varobj_iter_ctor.
	(py_varobj_iter_new): Remove.
	(py_varobj_get_iterator): Update.