It is actually unclear whether character references with more than 4
digits are valid. The XML spec seem to indicate they are but Xerces-C++
3.2 serializer throws an invalid character reference exception (the
parser appears to be able to handle them without problems).

Note that while we ignore wildcards and mixed content, their order information
is still saved.

anyType as a DOM fragment, similar to wildcards. anySimpleType as
a text string.

Add support for an implicit string pool in istream and ostream
templates. Use this support to pool type namespace and name
when dealing with polymorphic types.

Affected options: --generate-xml-schema, --export-maps,
--import-maps. New macros: XSD_NO_EXPORT, XSD_MAP_VISIBILITY.
Reorder template arguments for the idref type.

New option: --generate-detach. New test: cxx/tree/detach.

New options: --polymorphic-type, --polymorphic-type-all.

Now the default/fixed values are parsed by the compiler at compile time
instead of the standard parsing code at runtime.

Also add support for ISO-8859-1.