user_save() removed, use $account->save()

user_save() has been removed in favor of $account->save().

Code example

// Changing the username in Drupal 7.
$account = user_load(1);
$edit = array('name' => 'new_name');
user_save($account, $edit);

// Changing the username in Drupal 8.
$account = user_load(1);
$account->name = 'new_name';
$account->save();