Adds convenient methods for xpathEval()

[redvers]

Adds:

• xpathEvalNodes(xpath: String val, namespaces: Array[(String val, String val)] = []): Array[Xml2Node] ?
• xpathEvalString(xpath: String val, namespaces: Array[(String val, String val)] = []): String val ?
• xpathEvalF64(xpath: String val, namespaces: Array[(String val, String val)] = []): F64 ?
• xpathEvalBool(xpath: String val, namespaces: Array[(String val, String val)] = []): Bool ?

These functions are clearer in use at the call-site:

Compare:

  match doc.xpathEval("//child")
  | let nodes: Array[Xml2Node] => // etc…
  else
    h.fail("Expected nodeset result for //child")
  end

or

  let nodes: Array[Xml2Node] = (doc.xpathEval("//child") as Array[Xml2Node])

To:

  let nodes: Array[Xml2Node] = doc.xpathEvalNodes("//child")?

[redvers]

Hi @redvers,

The changelog - added label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 20.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.