async implementation re-design

[onur-ozkan]

If we can make AsyncMavConnection tokio-free and more generic, that would allow anyone with any async runtime (especially people who build on low-power devices) to use this trait, which would be amazing.

[onur-ozkan]

I am thinking about dropping the entire async implementation we have today (which duplicates everything in "mavlink-core/src/connection") and implement _async version of each sync function (behind a feature gate called "async") on MavConnection, e.g.:

diff --git a/mavlink-core/src/connection/mod.rs b/mavlink-core/src/connection/mod.rs
index 979d96b67..c7a79e142 100644
--- a/mavlink-core/src/connection/mod.rs
+++ b/mavlink-core/src/connection/mod.rs
@@ -34,6 +34,8 @@ pub trait MavConnection<M: Message> {
     /// return any errors, otherwise return any errors that occured while receiving.  
     fn recv(&self) -> Result<(MavHeader, M), MessageReadError>;
 
+    async fn recv_async(&self) -> Result<(MavHeader, M), MessageReadError>;
+
     /// Receive a raw, unparsed MAVLink message.
     ///
     /// Blocks until a valid frame is received, ignoring invalid messages.
@@ -44,6 +46,8 @@ pub trait MavConnection<M: Message> {
     /// return any errors, otherwise return any errors that occured while receiving.  
     fn recv_raw(&self) -> Result<MAVLinkMessageRaw, MessageReadError>;
 
+    async fn recv_raw_async(&self) -> Result<MAVLinkMessageRaw, MessageReadError>;
+
     /// Try to receive a MAVLink message.
     ///
     /// Non-blocking variant of `recv()`, returns immediately with a `MessageReadError`
@@ -61,6 +65,8 @@ pub trait MavConnection<M: Message> {
     /// This function will return a [`MessageWriteError::Io`] error when sending fails.
     fn send(&self, header: &MavHeader, data: &M) -> Result<usize, MessageWriteError>;
 
+    async fn send_async(&self, header: &MavHeader, data: &M) -> Result<usize, MessageWriteError>;
+
     /// Sets the MAVLink version to use for receiving (when `allow_recv_any_version()` is `false`) and sending messages.
     fn set_protocol_version(&mut self, version: MavlinkVersion);
     /// Gets the currently used MAVLink version
@@ -82,6 +88,10 @@ pub trait MavConnection<M: Message> {
         self.send(&frame.header, &frame.msg)
     }
 
+    async fn send_frame_async(&self, frame: &MavFrame<M>) -> Result<usize, MessageWriteError> {
+        self.send_async(&frame.header, &frame.msg)
+    }
+
     /// Read whole frame
     ///
     /// # Errors
@@ -97,6 +107,16 @@ pub trait MavConnection<M: Message> {
         })
     }
 
+    async fn recv_frame_async(&self) -> Result<MavFrame<M>, MessageReadError> {
+        let (header, msg) = self.recv_async()?;
+        let protocol_version = self.protocol_version();
+        Ok(MavFrame {
+            header,
+            msg,
+            protocol_version,
+        })
+    }
+
     /// Send a message with default header
     ///
     /// # Errors
@@ -107,6 +127,11 @@ pub trait MavConnection<M: Message> {
         self.send(&header, data)
     }
 
+    async fn send_default_async(&self, data: &M) -> Result<usize, MessageWriteError> {
+        let header = MavHeader::default();
+        self.send_async(&header, data)
+    }
+
     /// Setup secret key used for message signing, or disable message signing
     #[cfg(feature = "signing")]
     fn setup_signing(&mut self, signing_data: Option<SigningConfig>);

This could solve many things at once. I am hoping to send a P.o.C PR in a week or so.