XSLT 2.0 implementation

Support for XSLT 2.0 is dropped, in the sense that there is no longer a mode of operation that accepts XSLT 2.0 syntax while rejecting XSLT 3.0 syntax. The level of backwards compatibility in XSLT 3.0 is very high, so it should still be possible to run XSLT 2.0 stylesheets under Saxon without difficulty; the only real impact of this change is that it is not possible to run XSLT 2.0 conformance tests that expect all errors against the 2.0 specification to be detected.