MIR: Operations on tickets

• JOIN_TICKETS: pair ( ticket cty ) ( ticket cty ) : A — option ( ticket cty ) : A @sras
• READ_TICKET: ticket cty : A — pair address cty nat : ticket cty : A @sras
• SPLIT_TICKET: ticket cty : pair nat nat : A — option ( pair ( ticket cty ) ( ticket cty ) ) : A @sras
• TICKET: cty : nat : A — option ( ticket cty ) : A @sras