Add FeedbackFactory implementations for CrashFeedback, CombinedFeedback and DiffFeedback.

[djoooooe]

This allows using more complex feedback constructs in StdTMinMutationalStage without having to implement FeedbackFactory for them.

Example:

let diff_feedback = DiffFeedback::new("MyDiffFeedback", &observer_a, &observer_b, |a, b| {
    compare_observations(a, b)
}).unwrap();

let objective = feedback_or!(diff_feedback, CrashFeedback::new());

let min_stage = StdTMinMutationalStage::new(
    mutator,
    objective,
    1 << 10,
);

[addisoncrump]

Nice. Can we remove the type definitions for the default factories?

[djoooooe]

Nice. Can we remove the type definitions for the default factories?

I don't know, maybe someone else can comment on that...

[addisoncrump]

Ah, let me rephrase. Please remove the existing type declarations (e.g., type CrashFeedbackFactory) where you've just made it possible for the Feedbacks to produce themselves.

[djoooooe]

I removed type CrashFeedbackFactory, I don't see another type made obsolete by my changes.

[addisoncrump]

LGTM