refactor: remove composer and reader

Composers and readers did not work as expected. It is just not possible for
himalaya to spawn a command that spawns $EDITOR, piping and redirection cannot
satisfy all the needs. Either the $EDITOR does not spawn (hangs over), either
himalaya does not collect any output from edition. The simplest way is to use an
intermediate temp file, or use process substitution. For eg., using mml:

  mml compose >(himalaya message send)

You can also write into a file then feed himalaya with it.

src/imap/message/save.rs

@@ -46,7 +46,7 @@ pub struct ImapMessageSaveCommand {
 }
 
 impl ImapMessageSaveCommand {
-    pub fn execute(self, printer: &mut impl Printer, mut client: ImapClient) -> Result<()> {
+    pub fn execute(self, printer: &mut impl Printer, client: &mut ImapClient) -> Result<()> {
         let mailbox: Mailbox<'static> = self.mailbox.inner.try_into()?;
         let message = self.message.parse()?;
         let message = Literal::try_from(message)?;