Add rule unfold derived table

[jiangzhe]

This rule is to unfold flat derived table containing only simple projections.
For example:

SELECT * FROM (SELECT c0, c1 + 1 as cc FROM t1) x1 WHERE f(cc+1) > 0 

could be unfolded and translate to equivalent SQL like below:

SELECT c0, c1 + 1 as cc FROM t1 WHERE f(c1 + 1 + 1) > 0

The benefits are not only further optimizations like expression simplify, but also more tables involved in join reorder.
In some scenarios, users specify bushy join via derived table:

SELECT 1 FROM t1 JOIN t2 JOIN (SELECT * FROM t3 JOIN t4)

With this rule applied, we could reorder 4 tables instead of 3. Althrough SQL does not allow such syntax:

SELECT 1 FROM t1 JOIN t2 JOIN (t3 JOIN t4)

More advanced unfolding is left for future improvements, e.g. certain patterns of aggr+join involved in derived table could also be unfolded.

[jiangzhe]

There is limitation on outer query to apply this rule. Suppose we have SQL:

SELECT tt.c1, tt.c2 FROM t1 LEFT JOIN (SELECT 1 as c1, c2 FROM t2 JOIN t3) tt

Replacing topmost tt.c1 expression with 1 as c1 is absolutely wrong.
This is because the semantics of LEFT JOIN can change output values in right table to null.

That means, if any expression in derived table output can not ensure such behavior, we have to double check the outer query semantics, to ensure derived table is not at either sides of FULL JOIN or right side of LEFT JOIN.