Remove TypedConfigInterface::set()

OK, so let's roll #5 and #6 into one patch and then I'll RTBC :-)

+++ b/core/modules/config/src/Tests/ConfigSchemaTest.php
@@ -319,7 +313,7 @@ function testSchemaData() {
     // Finally update some object using a configuration wrapper.
     $new_slogan = 'Site slogan for testing configuration metadata';
     $wrapper = \Drupal::service('config.typed')->get('system.site');
-    $wrapper->set('slogan', $new_slogan);
+    \Drupal::configFactory()->getEditable('system.site')->set('slogan', $new_slogan)->save();
     $site_slogan = $wrapper->get('slogan');
     $this->assertEqual($site_slogan->getValue(), $new_slogan, 'Successfully updated the contained configuration data');

Let's just remove that whole hunk. It's also testing the method we're removing here.

Nope, that's perfect. Was about to RTBC but we need a beta evaluation here. Will do that tomorrow if no one beats me to it.

Re #19: Ahh, yes you're right. The first hunk that is removed from the test in fact tests the get() method, not the set() method. Yes, let's revert that commit and leave that part of the test in. Thanks, I had missed that!

Added a beta evaluation. Let's see what committers say. There is some theoretical disruption here, that can't be argued away, but I think it's really, really small, and the benefits greatly outweigh it in my opinion. We'll see.

Awesome, thanks!