Issue #1637478 by alexpott, pounard, catch: Add a PHP array cache backend.

core/lib/Drupal/Core/Cache/MemoryBackend.php

...es/system/lib/Drupal/system/Tests/Cache/CacheTestBase.php

...odules/system/lib/Drupal/system/Tests/Cache/ClearTest.php

...lib/Drupal/system/Tests/Cache/DatabaseBackendUnitTest.php

...al/system/Tests/Cache/GenericCacheBackendUnitTestBase.php

.../system/lib/Drupal/system/Tests/Cache/GetMultipleTest.php

...ules/system/lib/Drupal/system/Tests/Cache/IsEmptyTest.php

...m/lib/Drupal/system/Tests/Cache/MemoryBackendUnitTest.php

.../system/lib/Drupal/system/Tests/Cache/NullBackendTest.php

...dules/system/lib/Drupal/system/Tests/Cache/SavingTest.php