Add XPath function library and function interface
Create XPath function library

Remove hardcoding of function evaluation and introduce the false()
function.
Helper method to test if the library has a function
Ensure the core library has the correct functions
Pass the evaluation context to functions

This required forward declarations to avoid an include loop of
XPathFunction.
Evaluation context knows the size
Add equality method to XPath values
Add string output of XPath values
Add XPath function last()