Deprecate direct-access to alloc_f?

[jlnav]

Since all seem to fit either start_only_persistent, give_sim_work_first, or give_pregenerated_work.

The user could specify one of three policies, instead of importing/assigning a function.

The alloc_f could also maybe become a stateful class, similarly to to how gens are becoming classes, and how sims are also considered to become one.