Rename build/aux to build/ac-aux

The directory name 'aux' is special on Windows, and using it keeps the
build from working.

You will need to rerun autogen.sh after this change
Add span info to XML dump

For a node, add span information for the label, value, and node overall if
it is present in the tree. The resulting XML looks like

  <node label="..." file="..." path="...">
    <span for="label" start="..." end="..."/>
    <span for="value" start="..." end="..."/>
    <span for="node" start="..." end="..."/>
    <value>...</value>
  </node>