py/obj: Merge getiter and iternext mp_obj_type_t slots.

The goal here is to remove a slot (making way to turn make_new into a slot)
as well as reduce code size by the ~40 references to mp_identity_getiter
and mp_stream_unbuffered_iter.

This introduces two new type flags:
- MP_TYPE_FLAG_ITER_IS_ITERNEXT: This means that the "iter" slot in the
  type is "iternext", and should use the identity getiter.
- MP_TYPE_FLAG_ITER_IS_CUSTOM: This means that the "iter" slot is a pointer
  to a mp_getiter_iternext_custom_t instance, which then defines both
  getiter and iternext.

And a third flag that is the OR of both, MP_TYPE_FLAG_ITER_IS_STREAM: This
means that the type should use the identity getiter, and
mp_stream_unbuffered_iter as iternext.

Finally, MP_TYPE_FLAG_ITER_IS_GETITER is defined as a no-op flag to give
the default case where "iter" is "getiter".

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

tests/basics/io_stringio_base.py

@@ -0,0 +1,24 @@
+# Checks that an instance type inheriting from a native base that uses
+# MP_TYPE_FLAG_ITER_IS_STREAM will still have a getiter.
+
+try:
+    import uio as io
+except ImportError:
+    import io
+
+a = io.StringIO()
+a.write("hello\nworld\nmicro\npython\n")
+a.seek(0)
+
+for line in a:
+    print(line)
+
+class X(io.StringIO):
+    pass
+
+b = X()
+b.write("hello\nworld\nmicro\npython\n")
+b.seek(0)
+
+for line in b:
+    print(line)