A replacement of std.xml for the Phobos standard library

XML is probably the most used storage and interchange format for structured and semi-structured data, being also used for object serialization and configuration files. A good XML library is fundamental for many large applications; as such, shipping with a well-written std.xml would greatly benefit Phobos and more generally the D ecosystem. The goal of this project is to produce a library for handling the XML format and replace the current std.xml with it.