Create a commerce_wishlist_purchase field for wishlist items

The field instance needs to be called "purchases", with getPurchases() setPurchases() and addPurchase().

We should create a Purchase value object that has getOrderId() getQuantity() getPurchasedTime() methods, and is used by the getters/setters.

WishlistPurchasedFormatter should be WishlistPurchaseFormatter and it should output a table like the order item formatter.

+    $columns['purchased'] = [

Unsure about the column name. "timestamp" might be better. Ask Matt/Jonathan for a second opinion.

+function commerce_wishlist_update_8304() {
+  drupal_flush_all_caches();

Why do we need to flush caches at the beginning of this update function?
An irregular thing like that needs a comment with an explanation, assuming that it's actually needed.

+  public function getPurchases() {
+    return $this->get('purchases')->getValue();
+  }

Docs say that getPurchases will return an array of value objects. getValue() does not return an array of value objects.

Which brings me to tests. We need a unit test for the value object. A kernel test for wishlist items that will test the purchases methods.

+  public function __toString() {
+    $this->stringTranslation = \Drupal::service('string_translation');
+    /** @var \Drupal\Core\Datetime\DateFormatterInterface $date_formatter */
+    $date_formatter = \Drupal::service('date.formatter');
+    $vars = [
+      '@purchased' => $date_formatter->format($this->purchasedTime),
+      '@order_id' => $this->orderId,
+    ];
+    return $this->formatPlural($this->quantity, '@count item has been bought in order @order_id on @purchased.', '@count items have been bought in order @order_id on @purchased.', $vars);
+  }

Using \Drupal in a value object is icky. A __toString() method is meant for debugging purposes, not for returning a string that can be shown in a UI. I'd remove the entire __toString() method.

+  public function viewElements(FieldItemListInterface $items, $langcode) {
+    $elements = [
+      '#type' => 'table',
+      '#caption' => $this->t('Purchased items'),

Caption is incorrect. We're listing Purchases, not purchased items.

Sneaked in getPurchasedQuantity() and getLastPurchasedTime() order item methods.

Reverted purchased -> purchased_time rename, the original made more sense.
Fixed bug that was causing the Reports screen to always report the field as needing an update (due to the current time default value for "placed").
Other cleanups.