Issue #2112807 by tim.plunkett: Move the form builder functions in form.inc to a form service.

core/core.services.yml

core/includes/form.inc

core/includes/install.core.inc

core/lib/Drupal.php

core/lib/Drupal/Core/Controller/HtmlFormController.php

core/lib/Drupal/Core/Form/FormBuilder.php

core/lib/Drupal/Core/Form/FormBuilderInterface.php

core/modules/edit/lib/Drupal/edit/EditController.php

core/modules/file/file.module

...les/system/lib/Drupal/system/Tests/Form/FormCacheTest.php

core/tests/Drupal/Tests/Core/Form/FormBuilderTest.php

core/tests/Drupal/Tests/UnitTestCase.php