Simplify checkout with single address entry for shipping and billing

This very common cart feature was implemented in Commerce 1.x but did not make it to Commerce 2.x.
Basically if a shipping address is already recorded there should be a "same as shipping" checkbox to populate the billing address automatically instead of asking the user to re-enter the same address.

This does not seem too difficult to implement but would have a real benefit to the users.

This issue: https://www.drupal.org/node/2844920 probably takes care of a logged in user for reuse of profiles but the address copy would still be needed for anonymous checkout.