Tickets: Prepare MR for storage part

Introduces a storage module used for the ticket-balance table.

The new table is for tracking ticket ownership. It's a mapping from ticketer (the creator of the ticket), content and owner (the contract currently holding parts of the ticket type). For instance:

Ticketer x Content x Owner	Amount
(KT789, "T1", KT789)	1
(KT789, "T1", KT123)	5
(KT700, Unit, KT800)	7
...	...

The top-level API for modifying the table is exposed through a module Alpha_context.Ticket_balance.

It defines a type token which represents a ticket type (a pair of ticket creator and content).

It provides a function adjust_balance for updating the ticket-balance table:

val adjust_balance :
  t ->
  token:token ->
  owner:Contract.t ->
  delta:Z.t ->
  (t, error trace) result Lwt.t