Brassaia: Making Io a wrapper around Unix

What

Defunctorization of the modules depending on Io

Why

Io was a signature allowing to use the implemented Unix module or a custom one. Only Unix is used as a backend in octez, it makes no sense to keep the abstraction and it makes the code simpler.

How

Remove Io as a functor parameter in the rest of the codebase Remove some _intf files that are no longer useful

Manually testing the MR

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