Originally, we had enforced invariants only at calls to "normal"
methods, and excluded ``__setattr__`` since it is usually too expensive
to verify invariants whenever setting an attribute.

However, there are use cases where the users prefer to incur to
computational overhead for correctness. To that end, we introduced the
feature in this patch to steer when the invariants are enforced (at
method calls, on setting attributes, or in both situations).

Fixes #291.