Merge #613

613: Rust stable support r=Dirbaio a=Dirbaio

This PR adds (limited) stable Rust support!

The drawbacks are: 

- No `#[embassy::task]`, `#[embassy::main]`. (requires `type_alias_impl_trait`). You have to manually allocate the tasks somewhere they'll live forever. See [example](https://github.com/embassy-rs/embassy/blob/master/examples/nrf/src/bin/raw_spawn.rs)
- No async trait impls (requires GATs). Note that the full API surface of HALs is still available through inherent methods: #552 #581 
- Some stuff is not constructible in const (requires `const_fn_trait_bound`), although there's an (ugly) workaround for the generic `Mutex`.

So it's not that bad in the end, it's fully usable for shipping production-ready firmwares. We'll still recommend nightly as the default, until GATs and `type_alias_impl_trait` are stable.

Co-authored-by: Dario Nieuwenhuis <dirbaio@dirbaio.net>

embassy-nrf/src/spim.rs

@@ -374,7 +374,6 @@ mod eh02 {
 #[cfg(feature = "unstable-traits")]
 mod eh1 {
     use super::*;
-    use core::future::Future;
 
     impl embedded_hal_1::spi::Error for Error {
         fn kind(&self) -> embedded_hal_1::spi::ErrorKind {
@@ -437,7 +436,7 @@ mod eh1 {
 
         fn transaction<'a>(
             &mut self,
-            operations: &mut [embedded_hal_async::spi::Operation<'a, u8>],
+            operations: &mut [embedded_hal_1::spi::blocking::Operation<'a, u8>],
         ) -> Result<(), Self::Error> {
             use embedded_hal_1::spi::blocking::Operation;
             for o in operations {
@@ -451,6 +450,12 @@ mod eh1 {
             Ok(())
         }
     }
+}
+
+#[cfg(all(feature = "unstable-traits", feature = "nightly"))]
+mod eh1a {
+    use super::*;
+    use core::future::Future;
 
     impl<'d, T: Instance> embedded_hal_async::spi::Read<u8> for Spim<'d, T> {
         type ReadFuture<'a>