(re)Make `Slot_repr.t` abstract

Context

This MR abstracts Slot_repr base type.

This additionally expands the documentation of the interface of the module.

A unit test violating invariants of Slot_repr.t (aka consensus_committe_size must be < 2^16 since Slot_repr can be created from `round mod consensus_committe_size) has been adapted.

Fixes: #2057 (closed)

Manually testing the MR

Unit tests dune exec src/proto_alpha/lib_protocol/test/main.exe -- test "^baking$" should pass.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• [n/a] Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• [n/a] 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