Fixed #16905 -- Added extensible checks (nee validation) framework

This is the result of Christopher Medrela's 2013 Summer of Code project.

Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian
Apolloner, and Alex Gaynor for review notes along the way.

Also: Fixes #8579, fixes #3055, fixes #19844.

tests/model_fields/tests.py

@@ -85,6 +85,11 @@ class BasicFieldTests(test.TestCase):
         klass = forms.TypedMultipleChoiceField
         self.assertIsInstance(field.formfield(choices_form_class=klass), klass)
 
+    def test_field_str(self):
+        from django.utils.encoding import force_str
+        f = Foo._meta.get_field('a')
+        self.assertEqual(force_str(f), "model_fields.Foo.a")
+
 
 class DecimalFieldTests(test.TestCase):
     def test_to_python(self):