Add functions/types for XPath support.

The line length thing also applies to code lines ;-)
For wrapping inside `()` of a single logical line, we use 4 spaces of indentation. So it should be something like:
```scala
  def evaluate(xpathExpression: String, contextNode: Node,
      namespaceResolver: XPathNSResolver, resultType: Int,
      result: XPathResult): XPathResult = ???
```