Implement a method to compute state metadata from an Observer

[andreafioraldi]

At the moment, a Feedback can process an Observer and produce just testcase metadata.

As Feedback it related to the Corpus evolution, and as part of State cannot handle State for borrowing reasons, I propose to add state metadata based on observers in the post_exec() method of Executor.

We should create the MetaCollectorExecutor trait that override post_exec and forward it to an underlying base implementation.

Something like

let executor = DictionaryCollectorExecutor::new(InProcessExecutor::new(...));

[andreafioraldi]

This will imply that there will be some observers just for this that are never used in any feedback.
We should avoid to pack them when sending the NewTestcase event.

I'm thinking about different solutions to do this:

• define a SerializableObserversTuple as a tuple of Option and serialize to None the observers marked as not related to any feedback by the user or by the observer itself with a method
• before serializing, create a tuple of &Observer removing the observers marked as not related to any feedback by the user or by the observer itself with a static method
• the observer that know to not need any feedback simply serialize to NopObserver

[andreafioraldi]

Done with #84