TODO

* Remove Order, Reverse, Limit and Offset from the Relation inheritance chain
  * Instead make it so they are proxy classes that wrap Relations, but
    proxy method calls to the Relation methods, and then wrap the return
    value (assuming it's a Relation) in a copy of itself.
  * Also this will allow knowledge of #directions to be removed from
    all the Relation classes. Only the Ranking classes will need to know
    about directions.
  * The Reverse class will probably not be needed either. All we need is
    for the #reverse method to create a new Order with the directions
    reversed.
  * Remove the knowledge of directions from materialized relations
    and instead wrap materialized relations in the Order object
    if any.

* Add Relation#group and Relation#ungroup

* Update Attributes so that constraints are modelled using predicates,
  so that when "join-ability" is tested, the predicates can just be
  compared for equality.
  * Create an object that can wrap a predicate, and if it fails it will
    set the object's error state, and then propagate the status to the
    caller. This will allow a "complex" predicate to set the errors
    on tuples, while still allowing all the same objects to be used.

* Update memoization to take into account the arguments passed into
  the method. It should memoize differently for different
  methods. Perhaps it could have a special case for methods
  with a 0-arity vs n-arity methods, but it needs to handle cases
  like with #hash that accept no args, and #optimize that accepts
  1 arg.

* Add Relation#antijoin
  * Allows "NOT EXISTS/IN" type queries to be executes where every tuple
    from the relation is returned that *does not* join with the tuples in
    the operand.

* Inclusion/Exclusion improvements:
  * Allow attribute/function expressions to exist within the enumerable part
    of the Inclusion and Exclusion operations
    * The #call method should propagate the Tuple down to each callable entry
      in the Enumerable.

* Within the restriction context, make sure the attributes have aliases for:
  * Alias #== to #eq (may break alot)
  * Alias #!= to #ne (in 1.9 only)
  * Alias #>= to #gte
  * Alias #> to #gt
  * Alias #<= to #lte
  * Alias #< to #lt
  * Alias #=~ to #match

* Within the sort_by context, make sure the attributes have aliases for:
  * Alias #@+ to #asc
  * Alias #@- to #desc