Add ResourceObjectData, IncludedData, and RelationshipData specializations of Data

s/PrimaryData/ResourceObjectData/ because on relationship responses, the relationship data is the primary data.

+++ b/src/JsonApiResource/Data.php
@@ -10,7 +10,7 @@ use Drupal\jsonapi\Exception\EntityAccessDeniedHttpException;
-class Data implements \IteratorAggregate, \Countable {
+abstract class Data implements \IteratorAggregate, \Countable {

+++ b/src/JsonApiResource/IncludedData.php
@@ -0,0 +1,28 @@
+class IncludedData extends Data {
...
+  public function __construct($data) {
+    assert(Inspector::assertAllObjects($data, ResourceObject::class, EntityAccessDeniedHttpException::class));

+++ b/src/JsonApiResource/RelationshipData.php
@@ -0,0 +1,29 @@
+class RelationshipData extends Data {
...
+  public function __construct(array $data, $cardinality = -1) {
+    assert(Inspector::assertAllObjects($data, ResourceIdentifier::class));

+++ b/src/JsonApiResource/ResourceObjectData.php
@@ -0,0 +1,30 @@
+class ResourceObjectData extends Data {
...
+  public function __construct($data, int $cardinality = -1) {
+    assert(Inspector::assertAllObjects($data, ResourceObject::class, EntityAccessDeniedHttpException::class));

I do like the extra type safety this gets us! The fact that they're all still @internal means this is solely refactoring of implementation details that gets us type safety.

(We can nail the name later, before making it non-internal.)

This looks wonderful! Just one question: why not make each concrete Data subclass final?

RTBC as far as I'm concerned.

#3032679: Clean-up: rename EntityCollection to Data landed. Per #7, RTBC.

Re-uploading the #6 do-not-test patch. Will commit if tests pass.

Heh, forgot to upload.

🎉

This introduced a regression on PHP 5: #3035676: Follow-up for #3033123: PHP 5.5 + Drupal 8.7 test runs are suddenly failing.