`ActivePlan::mutators()` should return `Iterator<&mut Mutator>`.

[qinsoon]

We currently require the binding to implement get_next_mutator() and reset_mutator_iterator(), then we provide an implementation of mutators() -> SynchronizedMutatorIterator (which uses those two methods).

We could simply require the binding to implement mutators(), and change its return type to impl Iterator<&mut Mutator>. In that case, we no longer need get_next_mutator() and reset_mutator_iterator(). And API is more idiomatic in Rust.