Add support for fn:load-xquery-module

[GuntherRademacher]

These changes add support for fn:load-xquery-module. Besides the actual function implementation, the following had to be changed:

• QueryContext has a new member public SeqType contextType. This is also stored in StaticContext, but the sc of a parsed module was not accessible in the function implementation, where the context type is needed for type checking and coercion.
• Variables.bindExternal has a new flag cast, and it will cast values only if the flag is set, otherwise they will be coerced.
• the atomization of Options values has been removed. This was necessary to allow for node values being passed to the loaded module's external variables.

The test infrastructure had to be changed to

• support QName values of test-case/environment/param/@name,
• support location-to-file mapping via test-case/module/@location,
• remove fn-load-xquery-module from the list of non-supported features,
• skip tests explictly asking for no support of a feature that is actually supported.

There are 81 specific QT4 tests for fn:load-xquery-module, this implementation succeeds for all of them. Also some more tests are turning from failure to success, because they are affected by the changes made here.