SI-3368 CDATA gets a Node

XML Parser uses `scala.xml.PCData`.

A compiler flag `-Yxml:coalescing`, analogous to
`DocumentBuilderFactory.setCoalescing`, turns `PCData`
nodes into `Text` nodes and coalesces sibling text nodes.

This change also fixes parse errors such as rejecting a
sequence of CDATA sections.

A sequence of "top level" nodes are not coalesced.

```
scala> <a><b/>start<![CDATA[hi & bye]]><c/>world<d/>stuff<![CDATA[red & black]]></a>
res0: scala.xml.Elem = <a><b/>start<![CDATA[hi & bye]]><c/>world<d/>stuff<![CDATA[red & black]]></a>

scala> :replay -Yxml:coalescing
Replaying: <a><b/>start<![CDATA[hi & bye]]><c/>world<d/>stuff<![CDATA[red & black]]></a>
res0: scala.xml.Elem = <a><b/>starthi &amp; bye<c/>world<d/>stuffred &amp; black</a>
```