Many filters requires to keep a state within a session. Although it not
currently possible to do so, the callback now accept a mutable object of
type NoContext. This is the first step, the next one will be to accept
any type that implements both Clone and Default.
An other step will be to allow the user to define different callback
parameters depending on his/her needs.
The construction of an EventHandler object should not be directly done
by the client. Instead, it is easier to use procedural macro to
automatize the process, hence exposing a nice and simple interface. Such
use of procedural macros requires to crate an additional crate.
