#272 XmlValidation can now check against multiple schemas in a single document

Fortunately this functionality is already built into `StrictXML`, so I simplified our implementation by not having to manually specify a XSD. The catch is, the current implementation breaks #246, but it only applies in cases where IO problems occur, e.g. when a server is not accessible.

I believe the simpler implementation is preferable, and I added a puzzle so that we can fix it later. Please take it into account when reviewing.