Add `XmlMapper.createGenerator(XMLStreamWriter)` and `XmlMapper.createParser(XMLStreamReader)` overloads

[cowtowncoder]

Currently XmlMapper has convenience overrides for readValue() and writeValue() methods that take Stax reader/writer (XmlStreamReader, XmlStreamWriter), but no factory methods for creating generators or parsers -- these do exist in XmlFactory so it would be easy to delegate (plus they are actually needed for readValue()/writeValue() overrides).

So let's add overloads.