The simple_filter macro must accept the two different levels of
contexts. In the same way, it should also accept the log level, and
therefore replace the now removed simple_filter_log_level macro.
Threads are a bad idea because for now the filter API is not guaranteed
to be state-less. The interface is now synchronous, which should be
enough for most filters.
The refactoring brought other changes, the most important being the
concept of modular input sources and output destination and the complete
rewrite of the procedural macro.
The main goal of events/reports is to update a context object, which
will be used in filters to generate a response. It is now possible to
use any object implementing both Clone and Default as a context object.
It is also possible to define no context at all.
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.
