Reimplement parsing and storage of default expressions and constraint

expressions in CREATE TABLE.  There is no longer an emasculated expression
syntax for these things; it's full a_expr for constraints, and b_expr
for defaults (unfortunately the fact that NOT NULL is a part of the
column constraint syntax causes a shift/reduce conflict if you try a_expr.
Oh well --- at least parenthesized boolean expressions work now).  Also,
stored expression for a column default is not pre-coerced to the column
type; we rely on transformInsertStatement to do that when the default is
actually used.  This means "f1 datetime default 'now'" behaves the way
people usually expect it to.
BTW, all the support code is now there to implement ALTER TABLE ADD
CONSTRAINT and ALTER TABLE ADD COLUMN with a default value.  I didn't
actually teach ALTER TABLE to call it, but it wouldn't be much work.

src/include/access/tupdesc.h

@@ -6,7 +6,7 @@
  *
  * Copyright (c) 1994, Regents of the University of California
  *
- * $Id: tupdesc.h,v 1.24 1999/07/16 17:07:28 momjian Exp $
+ * $Id: tupdesc.h,v 1.25 1999/10/03 23:55:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -21,22 +21,20 @@
 typedef struct attrDefault
 {
 	AttrNumber	adnum;
-	char	   *adbin;
-	char	   *adsrc;
+	char	   *adbin;			/* nodeToString representation of expr */
 } AttrDefault;
 
 typedef struct constrCheck
 {
 	char	   *ccname;
-	char	   *ccbin;
-	char	   *ccsrc;
+	char	   *ccbin;			/* nodeToString representation of expr */
 } ConstrCheck;
 
 /* This structure contains constraints of a tuple */
 typedef struct tupleConstr
 {
-	AttrDefault *defval;
-	ConstrCheck *check;
+	AttrDefault *defval;		/* array */
+	ConstrCheck *check;			/* array */
 	uint16		num_defval;
 	uint16		num_check;
 	bool		has_not_null;