Make some PersistentVector's and APersistentVector.SubVector's internals public

This commit makes public the following items:

 * PersistentVector.Node,
 * PersistentVector.Node binary ctor,
 * PersistentVector.Node's edit and array fields,
 * PersistentVector.EMPTY_NODE,
 * PersistentVector's shift, root and tail fields,
 * APersistentVector.SubVector's v, start and end fields.

Signed-off-by: Rich Hickey <richhickey@gmail.com>