Proto: API for scanning values for tickets

Context

This is the second of a series of MRs for introducing a ticket accounting feature in the protocol as described in this document. The first MR is adding for storage modules.

This MR introduces a module Ticket_scanner that provides functions for checking if a type contains tickets and collecting all tickets from a value. The module [Ticket_scanner] introduces a type for storing existentially quantified tickets:

type ex_ticket =
  | Ex_ticket :
      'a Script_typed_ir.comparable_ty * 'a Script_typed_ir.ticket
      -> ex_ticket

As well as exposing a function for collecting all tickets of a given type and value:

val tickets_of_value :
  Alpha_context.context ->
  include_lazy:bool ->
  'a Script_typed_ir.ty ->
  'a ->
  (ex_ticket list * Alpha_context.context) tzresult Lwt.t

This function will be used for implementing the ticket-balance feature in subsequent MRs.

Manually testing the MR

No manual tests required.Unit tests for this MR are run with:

> dune exec src/proto_alpha/lib_protocol/test/main.exe -- test "^ticket scanner"

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• [x] Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• [x] For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR