add test for xsd.String:accept()
extract test helper functions so they can be reused by other tests
move tests for xsd.* types into tests.xsd subpackage
move bug fixes to the bottom of the changelog as they are not so important
correctly apply pattern restrictions for simple types
small cleanups for pattern checking in xsd.String, xsd.Decimal
additional test to verify that subclasses of xsd.String still accept string values