Represent BETWEEN as a special node type in raw parse trees.

Previously, gram.y itself converted BETWEEN into AND (or AND/OR) nests of
expression comparisons.  This was always as bogus as could be, but fixing
it hasn't risen to the top of the to-do list.  The present patch invents an
A_Expr representation for BETWEEN expressions, and does the expansion to
comparison trees in parse_expr.c which is at least a slightly saner place
to be doing semantic conversions.  There should be no change in the post-
parse-analysis results.

This does nothing for the semantic issues with BETWEEN (dubious connection
to btree-opclass semantics, and multiple evaluation of possibly volatile
subexpressions) ... but it's a necessary preliminary step before we could
fix any of that.  The main immediate benefit is that preserving BETWEEN as
an identifiable raw-parse-tree construct will enable better error messages.

While at it, fix the code so that multiply-referenced subexpressions are
physically duplicated before being passed through transformExpr().  This
gets rid of one of the principal reasons why transformExpr() has
historically had to allow already-processed input.

src/include/nodes/parsenodes.h

@@ -232,7 +232,11 @@ typedef enum A_Expr_Kind
 	AEXPR_DISTINCT,				/* IS DISTINCT FROM - name must be "=" */
 	AEXPR_NULLIF,				/* NULLIF - name must be "=" */
 	AEXPR_OF,					/* IS [NOT] OF - name must be "=" or "<>" */
-	AEXPR_IN					/* [NOT] IN - name must be "=" or "<>" */
+	AEXPR_IN,					/* [NOT] IN - name must be "=" or "<>" */
+	AEXPR_BETWEEN,				/* name must be "BETWEEN" */
+	AEXPR_NOT_BETWEEN,			/* name must be "NOT BETWEEN" */
+	AEXPR_BETWEEN_SYM,			/* name must be "BETWEEN SYMMETRIC" */
+	AEXPR_NOT_BETWEEN_SYM		/* name must be "NOT BETWEEN SYMMETRIC" */
 } A_Expr_Kind;
 
 typedef struct A_Expr