Provide generic BddVariableSet that does not necessarily rely on string names

[daemontus]

In some cases, it is useful to have structured "Bdd variable" object, such as an enum which specifies different types of variables used for encoding different components of the system. This is currently not supported, but could be probably added surprisingly easily. For backwards-compatibility, we can have a layer on top of the new implementation that exposes the string concretization.

[daemontus]

Some initial work has started in generic-bdd-variable-set.