Implement the latest JSONiq spec changes into Zorba

[Ghislain Fourny]

The following changes regarding the XQuery parser need to be implemented (if not already):

- JSON Navigation (empty sequence must be returned if the lhs is a node or an atomic).
- Check that $object() is equivalent to jn:keys($object), i.e., with key duplicate elimination, and $array() is equivalent to members($array).

As well as these regarding functions in both parsers:

- remove jn:is-null, jn:object
- relax signatures of libjn:descendant-objects, libjn:descendant-pairs, libjn:values, libjn:flatten, libjn:project to item*. new-version($s) is basically equivalent to: $s ! typeswitch ($$) case the-right-type() return old-one-item-version($$) default return ().
- Add libjn:descendant-arrays().
- Relax jn:size to accept empty sequence.
- Fix roundtrippability of decimals and doubles in jn:encode-for-roundtrip (decimals must be serialized with a dot and doubles with an e).

And also in both parsers:

- Check that the fixed corner cases of serialization are handled correctly in zorba (trailing zeros, absence of digits, leading +, which are allowed in the JDM but not in the JSON syntax that zorba should output).
- Relax EBV of sequences beginning with an object or array to be true

[Markos Zaharioudakis (markos-za)]

Ghislain, please make a separate bug for all issues regarding serialization and assign it to Nicolae, or whoever else you think appropriate, but not me.