ected to construct and---if necessary---destroy objects
there yourself, using the APIs on <code>UnsafeMutablePointer&lt;Element&gt;</code>.
Typical usage stores a count and capacity in <code>Header</code> and destroys
any live elements in the <code>deinit</code> of a subclass.
<strong>Note:</strong> Subclasses must not have any stored properties; any storage
  needed should be included in <code>Header</code>.
Create a new instance of the most-derived class, calling
<code>factory</code> on the partially-cons