Allow XPath expression to be compiled by the query language.  This allows rewriting to XPath 1.
XPath 2 expression are now rewritten into XPath 1, when needed.  The compare and numeric-compare functions are provided for comparison operators, such "a < b" => "compare(a, b) < 0".

Fixes #5

The XPathParser has issues.  Debug.Assert needs to be replaced and its complaining about some expressions.