Add distinct keyword as shorthand for group with all fields and no aggregate functions

[julianhyde]

Add distinct keyword as shorthand for group with all fields and no aggregate functions.

For example, the following are equivalent:

(*) Using `distinct`
from e in scott.emp
  yield {e.job, e.deptno}
  distinct;

(*) Using `group` with no aggregate functions
from e in scott.emp
  yield {e.job, e.deptno}
  group job, deptno;

(This is analogous to how, in SQL, SELECT DISTINCT job, deptno FROM emp is equivalent to SELECT job, deptno FROM emp GROUP BY job, deptno. SQL has a complication that (thankfully) Morel does not: you are allowed to use DISTINCT in the same query.)