XQXQ's use of HOF should be updated

[Chris Hillery]

The XQDoc for xqxq:prepare-main-module() says:

 : Note that parameters 2 and 3 should be declared as follows:
 : as function($url as xs:string, $entity as xs:string) as item()
 : as function($uri as xs:string, $entity as xs:string) as xs:string*
 : However Zorba's implementation of higher-order functions (HOF) is not
 : yet complete enough to allow for this. When Zorba's HOF implementation
 : is complete this function signature will be changed.

I believe the HOF implementation is in fact complete enough for this, so we should make that change as part of Zorba 3.0.

(Note that this bug will likely be obviated by other more extensive XQXQ API changes, but I wanted to file it so we didn't forget the issue.)