The previous project architecture was far too complicated and hard to
maintain. The new one is much more simple. Although procedural macros
are cools, they are a no-go on Rust-OpenSMTPD.
Reports and filter are implemented (except data-line) but untested.
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 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.
