Fix issue #989: Impossible to create array with > 100 items

Fixed issue 989: Impossible to create an object with an array field
of more than 100 elements.

This issue is due to the function agtype_build_list taking elements
as arguments. As the elements can be anything, expressions too,
they need to be processed by the transform phase and resolved in
the execution phase. As PG has a limitation of 100 function
arguments, this restricts the size of arrays built through
agtype_build_list.

The fix was to break up large lists into segments of 100 items or
less and then use the concatenation operator to join them in the
execution phase.

Added regression tests (thanks Taha!).