Handle constraintDecl and query-language-binding

[martindholmes]

The new <constraintDecl> element is going to carry an attribute @queryBinding, which needs to be handled. Since our Schematron is generated directly into the RELAX NG, we need to find a way to push that information out to the output in a manner which can be used later, e.g. when extracting Schematron. The suggestion is to to generate a processing instruction to do this. We also have to copy out the contents of the <constraintDecl>, which will be things like namespace declarations.

[martindholmes]

With @HelenaSabel and @sydb we talked about what should happen in the event that the upstream ODD and the customization ODD have a conflict with regard to variable or namespace declarations or @queryBinding, and the answer we seem to be settling on is that the downstream ODD would win out.

[sydb]

Note that handling of @queryBinding may turn out to be a moot point iff the Schematroners come up with a mechanism, first. See their # 52.