Indexing by arbitrary XPath expressions

[gburgett]

The Database class should allow setting indexes on arbitrary XPath expressions. The index should be specified by the XPath expression and a Class<T> object where T extends IComparable, Alternately future versions may allow specifying a Comparer object to perform the comparisons.
The XPath expression should select nodes that are convertible to the given Class. If any are not, those are simply not indexed, so the index is not necessarily a complete index.

The particular engine implementation will choose how it implements indexing. Some may choose to ignore indexes.