Add support for enhanced array operations from RethinkDB 1.6

[mfenniak]

prepend: prepends an element to an array append: appends an element to an array insertAt: inserts an element at the specified index spliceAt: splices a list into another list at the specified index deleteAt: deletes the element at the specified index changeAt: changes the element at the specified index to the specified value add: adds two arrays -- returns the ordered union mul: repeats an array n times difference: removes all instances of specified elements from an array count: returns the number of elements in an array indexesOf: returns positions of elements that match the specified value in an array isEmpty: check if an array or table is empty setInsert: adds an element to a set setUnion: returns the union of two sets setDifference: returns the difference of two sets setIntersection: finds the intersection of two sets slice: extract part of an array nth: extract an element from an array