Real before_each and after_each

[mackwic]

Currently, our before and after hooks doesn't traverse the tree to the children, that is, a before will be executed once for any context child, would it be an example or an example group. The examples inside the example group would then have only one execution of the hook, which is awkward.

As before_each is necessarily FnMut, I don't know how to put it in the runner and respect the aliasing rules. I tried many times in many ways, and I had to resign.

Help wanted, please contact me if the topic interest you.

[regexident]

Implemented in #30