FR: as_xml()

[jennybc]

It would be awesome to have as_xml() to enable a roundtrip with as_list().

as_list() has a limited scope and that would be just fine for the inverse. I'm really only interested in being able to create XML from simple R lists.

as_list currently only handles the four most common types of children that an element might have:

• Other elements, converted to lists.
• Attributes, stored as R attributes.
• Text, stored as a character vector.

The XML package sort of has (had?) this, in the form of XML::toHTML(). This topic has also seen quite a bit of stackoverflow action: How to create xml from R objects, e.g., is there a 'listToXml' function?.