XML writer

[ianprime0509]

The current writer API is missing some desirable features to consider it fully "ready":

• Ensure all node types are covered (missing comments and CDATA currently)
• Attribute functions that don't require the entire value to be passed at once
• Safety checks for using the API wrong
• Writer functions for extended nodes (e.g. for text nodes, a function that returns a std.io.Writer which writes into the text node)
• Writer function to add a trailing newline to a document without reaching into the underlying sink
• Tests
• Documentation