SCORU: Add PVM computational model

Context

This MR introduces the PVM execution model:

• It introduces a concept of "tick" to count rollup execution step.
• It extends the PVM module type to specify what is needed for a PVM to be used by the arbiter.
• It implements a toy execution model in the Sc_rollup_arith module.

Fixes #2100 (closed)

Manually testing the MR

 dune exec src/proto_alpha/lib_protocol/test/pbt/test_sc_rollup_tick_repr.exe
 dune exec src/proto_alpha/lib_protocol/test/unit/main.exe -- test "^\[Unit\] sc rollup arith$"

Checklist

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