Add Relational.only function, which allows scalar sub-queries

[julianhyde]

Add Relational.only function, of type α list → α, which returns the sole element of a list, and throws if the list is empty or has more than one element.

The equivalent of the SQL query

SELECT e.ename,
  (SELECT d.dname FROM depts AS d WHERE d.deptno = e.deptno) AS dname
FROM emps AS e

will be

from e in emps
  yield {ename = e.name,
         dname = only (from d in depts
                       where d.deptno = e.deptno
                       yield d.name)}

[julianhyde]

Fixed in bb060be.