Add a test to simulate integration into built-in actors

The built-in actors are the first and currently only consumer of these libraries (though of course we don't expect that to continue). Ease of integration there is important. However that environment has it's own imperfect design choices, e.g. the `Runtime` trait that abstracts over the FVM syscalls and presents a mockable interface for tests.

We could add some test code that starts with a similar Runtime interface, then adapts the token library (`ActorRuntime`) to it and uses it for some basic actor operation. Then we can see the static impact of API changes immediately, without going to the effort of a trial integration into built-in actors every time we want to improve something, and avoiding potential impedance mismatch when we later integrate.

FYI @Stebalien @arajasek do you think this could help?