Rewrite the project

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.

src/data_structures/auth_result.rs

@@ -0,0 +1,31 @@
+use std::str::FromStr;
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub enum AuthResult {
+    Pass,
+    Fail,
+    Error,
+}
+
+impl ToString for AuthResult {
+    fn to_string(&self) -> String {
+        match self {
+            AuthResult::Pass => String::from("pass"),
+            AuthResult::Fail => String::from("fail"),
+            AuthResult::Error => String::from("error"),
+        }
+    }
+}
+
+impl FromStr for AuthResult {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            "pass" => Ok(AuthResult::Pass),
+            "fail" => Ok(AuthResult::Fail),
+            "error" => Ok(AuthResult::Error),
+            _ => Err(()),
+        }
+    }
+}