Basic types for representing XML.

The idea is to have a full set of appropriate types, which various XML libraries can share. Instead of having equivalent-but-incompatible types for every binding, parser, or client, they all share the same types can can thus interoperate easily.

This library contains complete types for most parts of an XML document, including the prologue, node tree, and doctype. Some basic combinators are included for common tasks, including traversing the node tree and filtering children.