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/io.rs

@@ -0,0 +1,46 @@
+use std::io::{self, ErrorKind, Read};
+use std::sync::mpsc::Sender;
+
+pub(crate) fn read_stdin(tx: &Sender<Vec<u8>>) {
+    if let Err(e) = do_read_stdin(tx) {
+        log::error!("{}", e);
+    }
+}
+
+fn do_read_stdin(tx: &Sender<Vec<u8>>) -> Result<(), String> {
+    let mut read_buffer: [u8; crate::BUFFER_SIZE] = [0; crate::BUFFER_SIZE];
+    let mut line_buffer: Vec<u8> = Vec::with_capacity(crate::BUFFER_SIZE);
+    let mut stdin = io::stdin();
+    loop {
+        read_buffer.copy_from_slice(&[0; crate::BUFFER_SIZE]);
+        let len = match stdin.read(&mut read_buffer) {
+            Ok(n) => n,
+            Err(e) => match e.kind() {
+                ErrorKind::Interrupted => {
+                    continue;
+                }
+                _ => {
+                    return Err(e.to_string());
+                }
+            },
+        };
+        if len == 0 {
+            return Err(String::from("unable to read on stdin"));
+        }
+        line_buffer.extend_from_slice(&read_buffer);
+        loop {
+            match line_buffer.iter().position(|i| *i == b'\n') {
+                Some(id) => {
+                    let pos = id + 1;
+                    let mut line = Vec::with_capacity(pos);
+                    line.extend_from_slice(&line_buffer[..pos]);
+                    tx.send(line).unwrap();
+                    line_buffer.drain(..pos);
+                }
+                None => {
+                    break;
+                }
+            };
+        }
+    }
+}