Accessing the original entity in field templates is hard

Problem/Motivation

Let's make a field template and access some date from the entity:

datetime = element['#object'].getCreatedTime|format_date('html_datetime')

That's less than ideal.

Proposed resolution

Provide at least entity by adding it in template_preprocess_field

$variables['entity'] = $element['#object'];

Remaining tasks

User interface changes

API changes

Data model changes