Issue #1854868 by swentel: Let formatter summaries return arrays instead of strings.

...etime/Plugin/field/formatter/DatetimeDefaultFormatter.php

...Plugin/field/formatter/EntityReferenceEntityFormatter.php

.../Plugin/field/formatter/EntityReferenceLabelFormatter.php

.../lib/Drupal/field/Plugin/Type/Formatter/FormatterBase.php

...Drupal/field/Plugin/Type/Formatter/FormatterInterface.php

.../modules/field/tests/modules/field_test/field_test.module

...test/Plugin/field/formatter/TestFieldDefaultFormatter.php

...est/Plugin/field/formatter/TestFieldMultipleFormatter.php

.../Plugin/field/formatter/TestFieldPrepareViewFormatter.php

core/modules/field_ui/field_ui.api.php

.../modules/field_ui/lib/Drupal/field_ui/DisplayOverview.php

...ib/Drupal/image/Plugin/field/formatter/ImageFormatter.php

.../lib/Drupal/link/Plugin/field/formatter/LinkFormatter.php

.../number/Plugin/field/formatter/DefaultNumberFormatter.php

...rupal/picture/Plugin/field/formatter/PictureFormatter.php

...lephone/Plugin/field/formatter/TelephoneLinkFormatter.php

...upal/text/Plugin/field/formatter/TextTrimmedFormatter.php