SCORU: Add a STORE instruction to the arithmetic PVM

Context

To ease testing and demonstration, we extend the arithmetic PVM with a STORE instruction which allows any intermediate integer to be stored in the rollup state.

Manually testing the MR

Observe that the new tests using the STORE instruction pass.

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