Proto/block votes: parameterize EMA update computation

Context

This MR turns the Toggle_EMA module into a functor. It is instantiated twice in the Toggle_votes module: one instance for Liquidity Baking and one instance for Adaptive Inflation.

Manually testing the MR

The existing Liquidity Baking tests should still pass:

$ dune exec src/proto_alpha/lib_protocol/test/unit/main.exe -- --file test_liquidity_baking_repr.ml
$ dune exec src/proto_alpha/lib_protocol/test/integration/main.exe -- --file test_liquidity_baking.ml

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