Add "group" and "compute" clauses to "from" expression, aggregation and aggregate functions

[julianhyde]

Add "group" and "compute" clauses to "from" expression. The effect is aggregation and aggregate functions. With "group" you cannot use "yield", only "compute".

For example, the expression

from e in emps
group (#deptno e) as deptno
  compute sum of (#id e) as sumId,
          count of e as count

is equivalent to the SQL query

SELECT e.deptno, SUM(e.id) AS sumId, COUNT(*) AS "count"
FROM emps
GROUP BY e.deptno

[julianhyde]

Fixed in f12d9cb.

There is a follow-up task to allow aggregate functions defined in the query. (The type inference algorithm needs to be made a bit more robust.)