Add entity_access in core.

[klonos]

This was mentioned in backdrop-ops/contrib#71 (comment)

entity_access

I'm not sure how comprehensive this function is. Access checking seems like a good addition to our API as well, but I don't think we have this integrated in core yet. Another good core candidate though.

[btopro]

very simple addition which would definitely ease migration since entity is a critical piece of most D7 anythings.

/**
 * Determines whether the given user can perform actions on an entity.
 *
 * For create operations, the pattern is to create an entity and then
 * check if the user has create access.
 *
 * @code
 * $node = entity_create('node', array('type' => 'page'));
 * $access = entity_access('create', 'node', $node, $account);
 * @endcode
 *
 * @param $op
 *   The operation being performed. One of 'view', 'update', 'create' or
 *   'delete'.
 * @param $entity_type
 *   The entity type of the entity to check for.
 * @param $entity
 *   Optionally an entity to check access for. If no entity is given, it will be
 *   determined whether access is allowed for all entities of the given type.
 * @param $account
 *   The user to check for. Leave it to NULL to check for the global user.
 *
 * @return boolean
 *   Whether access is allowed or not. If the entity type does not specify any
 *   access information, NULL is returned.
 *
 * @see entity_type_supports()
 */
function entity_access($op, $entity_type, $entity = NULL, $account = NULL) {
  if (($info = entity_get_info()) && isset($info[$entity_type]['access callback'])) {
    return $info[$entity_type]['access callback']($op, $entity, $account, $entity_type);
  }
}

[klonos]

...closing this in favor of #3011