Implement support for DEFAULT values for columns in tables

CREATE TABLE T(a INT, b INT NOT NULL);

-- The default default is NULL
INSERT INTO T VALUES (DEFAULT, 0);
INSERT INTO T(b)  VALUES (1);
SELECT * FROM T;
(NULL, 0)
(NULL, 1)

-- Adding a default to a table with rows, sets the values for the
-- existing rows (exist default) and new rows (current default).
ALTER TABLE T ADD COLUMN c INT DEFAULT 5;
INSERT INTO T VALUES (1, 2, DEFAULT);
SELECT * FROM T;
(NULL, 0, 5)
(NULL, 1, 5)
(1, 2, 5)