Add `Relational` structure

[julianhyde]

Add Relational structure. It is not part of the Standard ML Basis library, but contains various functions and types that are useful for relational query evaluation.

signature RELATIONAL =
sig
  (* Values of the "descending" type sort in reverse order to the type that
     they wrap. Thus 'order DESC i' sorts elements in the opposite direction to
     'order i'. *)
  datatype descending = DESC of 'a

  (* "compare (x, y)" returns LESS, EQUAL, or GREATER according to
   * whether its first argument is less than, equal to, or greater
   * than the second.
   *
   * Comparisons are based on the structure of the type
   * α. Primitive types are compared using their natural order;
   * Option types compare with NONE last; Tuple types compare
   * lexicographically; Record types compare lexicographically, with
   * the fields compared in alphabetical order; List values compare
   * lexicographically; Bag values compare lexicographically, the
   * elements appearing in an order that is arbitrary but is
   * consistent for each particular value. *)
  val compare : α * α → order

  (* "count list" returns the number of elements in list. Often used
   * with `group`, for example `from e in emps group e.deptno compute
   * countId = count`. *)
  val count : int list → int

  (* "empty list" returns whether the list is empty, for example `from
   * d in depts where empty (from e where e.deptno = d.deptno)`. *)
  val empty : α list → bool

  (* "max list" returns the greatest element of list. Often used with
   * `group`, for example `from e in emps group e.deptno compute maxId
   * = max over e.id`. *)
  val max : α list → α

  (* "min list" returns the least element of list. Often used with
   * `group`, for example `from e in emps group e.deptno compute minId
   * = min over e.id`. *)
  val min : α list → α

  (* "nonEmpty list" returns whether the list has at least one
   * element, for example `from d in depts where nonEmpty (from e
   * where e.deptno = d.deptno)`. *)
  val nonEmpty : α list → bool

  (* "only list" returns the sole element of `list`, for example `from
   * e in emps yield only (from d where d.deptno = e.deptno)`. Raises
   * `Empty` if the list does not have exactly one element. *)
  val only : α list → α

  (* "e elem collection" returns whether `e` is a member of
   * `collection`. *)
  val elem : α * α bag → bool | α * α list → bool

  (* "e notelem collection" returns whether `e` is not a member of
   * `collection`. *)
  val notelem : α * α bag → bool | α * α list → bool

  (* "sum list" returns the sum of the elements of `list`. Often used
   * with `group`, for example `from e in emps group e.deptno compute
   * sumId = sum over e.id`. *)
  val sum : int list → int
end

[julianhyde]

There is a partial fix in commit d7e601a.