Track typmods in plpgsql expression evaluation and assignment.

The main value of this change is to avoid expensive I/O conversions when
assigning to a variable that has a typmod specification, if the value
to be assigned is already known to have the right typmod.  This is
particularly valuable for arrays with typmod specifications; formerly,
in an assignment to an array element the entire array would invariably
get put through double I/O conversion to check the typmod, to absolutely
no purpose since we'd already properly coerced the new element value.

Extracted from my "expanded arrays" patch; this seems worth committing
separately, whatever becomes of that patch, since it's really an
independent issue.

As long as we're changing the function signatures, take the opportunity
to rationalize the argument lists of exec_assign_value, exec_cast_value,
and exec_simple_cast_value; that is, put the arguments into a saner order,
and get rid of the bizarre choice to pass exec_assign_value's isNull flag
by reference.

src/pl/plpgsql/src/plpgsql.h

@@ -226,6 +226,7 @@ typedef struct PLpgSQL_expr
 	Expr	   *expr_simple_expr;		/* NULL means not a simple expr */
 	int			expr_simple_generation; /* plancache generation we checked */
 	Oid			expr_simple_type;		/* result type Oid, if simple */
+	int32		expr_simple_typmod;		/* result typmod, if simple */
 
 	/*
 	 * if expr is simple AND prepared in current transaction,