Node entity is now a classed object

Backdrop now has a consistent way of performing all CRUD functions for entities, as well as accessing entity properties such as label, revision, id, etc.

This applies to:
Taxonomy Term (and Vocabulary) entities
User
Node
File

Example implementation:
Create a Node entity:

    $node = entity_create('node', array(
      'uid' => 1,
      'type' => 'post',
      'title' => 'Test node',
      'status' => 1,
      'comment' => 2,
      'promote' => 0,
      'sticky' => 0,
      'langcode' => LANGUAGE_NONE,
      'created' => REQUEST_TIME,
      'changed' => REQUEST_TIME,
    ));
    $node->save();
    $nid = $node->nid;

Calls to node_save() have been replaced in core with $node->save() (although node_save() continues to exist as a wrapper).