Heat the raspberries with water, sugar, and lemon juice. Dissolve the corn starch in 1/4 cup cold water. Stic into raspberries and cook until thickened and clear. Cool. Melt the marshmallows in milk and cool. Whip the cream and fold into the marshmallow mixture. Mix the graham cracker crumbs, nuts, and butter in a 9 x 13" pan. Press firmly into the bottom of the pan. Spread the marshmallow cream mixture over the crumbs. Spread the raspberry mixture over the top. Refrigerate until firm.