[Cleanup] Refactor interesting functions away from mixins

[mike-north]

Currently there are places where functions live on mixins for little or no reason (there may have been very good reasons before, but no longer).

While leaving the public API surface intact, we should refactor these out so that they may be consumed directly

Example

Before:

export default Mixin.extend({
  add(x) {
    return this.get('total') + x;
  }
});

After:

export function add(obj, x) {
  return obj.get('total') + x;
}

export default Mixin.extend({
  add(x) {
    return add(this, x);
  }
});

[marcoow]

mixins are deprecated now – see #2198