Mark simple node navigation functions as const
Mark node foreach navigation functions as const
Mark node output functions as const
Remove C-string from text node
Remove C-string from element
Remove C-string from comment
Create an attribute node
Move attribute output into the attribute class
Add ability to enumerate an elements attributes
Tokenize the at sign
Support at-sign abbreviation for attributes

This introduces the concept of principal node types for an axis.
Parse descendant-or-self axis
Add test for node-test attribute
Parse attribute axis
Include "element" in string form of element node test
Parse node node-test
Remove a use of raw pointer
Remove unused test helper methods
Add to_stream for text nodes