Due to the fact, that the after compile hook was generated into each module, and used
the modules attributes directly, from dialyzers point of view, all the branches were
based on constant values and there was no way that ever any condition changes.

This is solved by moving the implementation of the __after_compile__/2 hook into the
Que.Worker module, where it accesses the passed in environment to check the validity
of the freshly compiled module.

As a side effect, this also reduces the worker modules sizes slightly, as the __after_compile__/2
does not have to be compiled into each module anymore, when its sole purpose on runtime
is to only return nil!

Fix #21