Extend the runtime generator to allow complex types

[Gabriel Petrovay]

In the specification of the function iterators.

Some XQuery functions might allow only some complex types. This is currently the case for
- fn:serialize -> the 2nd parameter
- fn:analyze-string -> the return value

And it can probably be needed by other future zorba functions.

A declaration like this should be possible:

 <zorba:param>element(output:serialization-parameters)?</zorba:param>

[Sorin Marian Nasoi (sorin.marian.nasoi)]

Marked with Resolution = None at Dana's request.