[common] generics, not Any for ComparisonExpression.

[stonier]

Relates to #336.

Not entirely happy with this, but at least it removes the viral ramifications of Any.

One or Two Generics?

NB: if using both T and V:

    def __init__(
        self,
        variable: str,
        value: T,
        operator: typing.Callable[[T, V], bool]
    ):

this starts to get awkward. The user then must always place the value in the right hand side of the operator when checking. This is somewhat arbitrary.

If this is needed, we should rethink the way ComparisonExpression is handled (perhaps provide a method that facilitates this).

Covariant/Contravariant?

Certainly could be useful to widen the generics in this way. However, both ways make sense? Ultimately it depends on what the user's base and derived classes look like. There might be some common patterns with standard data structures that could be usefully accommodated though. However, since that is not known at this point, this PR is not making a decision on it yet.