feat: classes for inference systems and logical equivalence

[fmontesi]

This PR is the beginning of work for formalising general concepts about logic in CSLib, in order to streamline the development of logics. A major challenge is that judgements, proof systems, and semantics can be very different across logics, so we cannot assume any particular shapes for them.
To address this, this PR introduces:

• A class for inference systems, which gives access to the ⇓ notation for derivations and the associated notion of Derivability.
• A class of heterogeneous contexts (HasHContext), which allows for defining contexts that, when filled with a value of type T, yield something of a different type than T. This is useful for formalising judgemental contexts: a context with a hole that can be filled with a proposition, resulting in a judgement.
• A class for LogicalEquivalences. We formalise a logical equivalence as a congruence on propositions that preserves validity under any judgemental context.
• Instances of these concepts for HML and CLL, which serve as examples of very different structures.

Depends on #393 for the new context fill notation.

[chenson2018]

Just some nitpicks, I haven't properly thought about the big picture here yet.

[fmontesi]

Should be ok now, but we should merge #393 first.

[chenson2018]

Should be ok now, but we should merge #393 first.

I have approved 393 and will take a look at your most recent comments/changes tomorrow.

[chenson2018]

Holding off approval until the documentation issue mentioned at #CSLib > `Context.fill` notation @ 💬 is resolved, but this all seems fine to me.