Avoid recursion when processing simple lists of AND'ed or OR'ed clauses.

Since most of the system thinks AND and OR are N-argument expressions
anyway, let's have the grammar generate a representation of that form when
dealing with input like "x AND y AND z AND ...", rather than generating
a deeply-nested binary tree that just has to be flattened later by the
planner.  This avoids stack overflow in parse analysis when dealing with
queries having more than a few thousand such clauses; and in any case it
removes some rather unsightly inconsistencies, since some parts of parse
analysis were generating N-argument ANDs/ORs already.

It's still possible to get a stack overflow with weirdly parenthesized
input, such as "x AND (y AND (z AND ( ... )))", but such cases are not
mainstream usage.  The maximum depth of parenthesization is already
limited by Bison's stack in such cases, anyway, so that the limit is
probably fairly platform-independent.

Patch originally by Gurjeet Singh, heavily revised by me

src/include/nodes/parsenodes.h

@@ -225,9 +225,6 @@ typedef struct ParamRef
 typedef enum A_Expr_Kind
 {
 	AEXPR_OP,					/* normal operator */
-	AEXPR_AND,					/* booleans - name field is unused */
-	AEXPR_OR,
-	AEXPR_NOT,
 	AEXPR_OP_ANY,				/* scalar op ANY (array) */
 	AEXPR_OP_ALL,				/* scalar op ALL (array) */
 	AEXPR_DISTINCT,				/* IS DISTINCT FROM - name must be "=" */