Make `bdd_topvar()`, `bdd_minvar()`, and `bdd_maxvar()` not throw on terminals

[ssoelvsten]

It is kinda odd that bdd_topvar() and its siblings is not safe for terminals. We should make it return something more meaningful. One of the following two would work, I see arguments for both.

• Return bdd::max_label+1 in this case (bdd::pointer_type::terminal_level)
• Change the return type to optional<bdd::label_type> and return nothing in this case.

I prefer the prior, since it is not a breaking change.