Add flatten, groupLeft, groupRight, assocLeft, assocRight

Created by: aerabi

• flatten, [[1, 2], 3, [4, 5, 6]] |-> [1, 2, 3, 4, 5, 6]
• groupLeft, [1, 2, 3] |-> [[1, 2], 3]
• groupRight, [1, 2, 3] |-> [1, [2, 3]]
• assocLeft, [1, [2, 3]] |-> [[1, 2], 3]
• assocRight, [[1, 2], 3] |-> [1, [2, 3]]