Add Tickets (improved stamps)

This MR adds 4 instructions and one type to Michelson:

• TICKET 'a nat -> creates a ticket attached to the address SELF and with an amount in nat, returns a handle
• PUNCH handle -> punches the ticket, fails if the ticket is already punched
• READ handle : (address, 'a, nat) -> gives you back the content of the ticket
• SPLIT ('a handle) nat nat : pair handle 'a handle 'a -> splits a handle into two, punches the old one and verify that the sum of amount matches

With this, it is possible to easily emulate transferable permissions (create a Ticket, send it to someone), do multi-sig with multiple contracts (check Tickets) or embed UTXOs in Tezos.

I am mostly looking for feedback on the design at this stage.

This MR makes !84 (closed) obsolete.