Use procedural macros to define events

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.

opensmtpd/Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+name = "opensmtpd"
+version = "0.1.0"
+authors = ["Rodolphe Bréard <rodolphe@what.tf>"]
+edition = "2018"
+description = "Interface for OpenSMTPD filters"
+keywords = ["opensmtpd", "filter", "mail"]
+documentation = "https://docs.rs/opensmtpd/"
+repository = "https://github.com/breard-r/rust-opensmtpd"
+readme = "README.md"
+license = "CECILL-B"
+include = ["src/**/*", "Cargo.toml", "Licence_*.txt"]
+
+[dependencies]
+log = "0.4"
+nom = "4.1"
+opensmtpd_derive = { path = "../opensmtpd-derive", version="0.1" }
+
+[[example]]
+name = "dummy"
+path = "examples/dummy.rs"
+
+[dev-dependencies]
+env_logger = "0.6"