Add sort_array function

This PR adds Hive's ``sort_array`` function that sorts the given array in ascending order according to the natural ordering of its elements (only if the array elements are Comparable).
@cberner This doesn't compare the stack representations. Before the ``sort()`` call the stack representation is parsed to create a list of objects with the ``readValue()`` call, and the comparison is done on those objects. For me to better understand the issue, can you please take a look at my unit tests and construct some examples that won't work?