XPath 3.0 implementation

The EQName syntax has been revised, implementing the change agreed for bug 15399: it is now Q{uri}local.

Affecting XPath 2.0 also, some very long-standing bugs have been fixed in the handling of schema-validated documents using xsi:nil - specifically, ensuring that the typed value of a nilled element is always an empty sequence, getting the matching of types element(N, T) and element(N, T?) right, as well as the impact of nillability on type subsumption.