Optimize stack usage of Dispatch::poll

Dispatch::poll has to copy the request so that the app can write the
response to the interchange.  Previously, we called
Responder::take_request to obtain the message out of the interchange.
This implementation had a stack usage of 15280 bytes while the message
size is only 7609 bytes.

With this patch, we only request a reference from the responder and
manually copy the message to a buffer.  This reduces the stack size to
7664 bytes.

I have no idea why the take_request implementation creates an additional
copy of the message.  But as this function is the root for all CTAPHID
commands, this means we can save around 7 kB stack virtually everywhere.
Similar optimizations may be possible in other functions too.

CHANGELOG.md

@@ -5,6 +5,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+- Optimize stack usage of `Dispatch::poll`
 
 ## [0.1.1] - 2022-08-22
 - adjust to `interchange` API change

src/dispatch.rs

@@ -106,14 +106,17 @@ impl<'pipe, 'interrupt> Dispatch<'pipe, 'interrupt> {
 
     #[inline(never)]
     pub fn poll(&mut self, apps: &mut [&mut dyn App<'interrupt>]) -> bool {
-        let maybe_request = self.responder.take_request();
-        if let Some((command, message)) = maybe_request {
+        // We could call take_request directly, but for some reason this doubles stack usage.
+        let mut message_buffer = Message::new();
+        if let Ok((command, message)) = self.responder.request() {
             // info_now!("cmd: {}", u8::from(command));
             // info_now!("cmd: {:?}", command);
 
-            if let Some(app) = Self::find_app(command, apps) {
+            message_buffer.extend_from_slice(message).unwrap();
+
+            if let Some(app) = Self::find_app(*command, apps) {
                 // match app.call(command, self.responder.response_mut().unwrap()) {
-                self.call_app(*app, command, &message);
+                self.call_app(*app, *command, &message_buffer);
             } else {
                 self.reply_with_error(Error::InvalidCommand);
             }