Implement support for XML

[beikov]

I'd like to add the following functions for XML

• cast
• xpath

Similar to the JSON support #841 I'd also like to add support for path navigation, but this will also require quoting support in the parser as xml allows a dash in tag and attribute names. By default, a path dereference is interpreted as access the subtag I.e. accessing a tag attribute has to be done explicitly or when a java metamodel for the xml structure is available, we can use that to interpret this, though there is still the possibility of a conflict so we might want to think of a special syntax support since we already will add support quoted identifiers. Maybe attribute names like @name can be interpreted as xml tag attribute names?

Using xpath multiple times in a more complex expression could become inefficient, so we might want to consider wrapping xmlish expressions in a special function that takes care of optimizing this.

I'd also like to support selecting xml objects that are then deserialized to the target type with e.g. JAXB.

This is kind of related to #161 but not necessarily dependent.