async: disable Send lint in async traits

Adds `#[allow(async_fn_in_trait)]` to disable the lint in `async` trait
functions recommending `Send` in the return type.

Adding the type annotation is currently unstable, has little-to-no
utility in current crates using `embedded-hal`, and will break those users.

See
<#515 (comment)>
for details and discussion.

Author: rmsyn

embedded-hal-async/src/lib.rs

@@ -6,6 +6,8 @@
 // We don't immediately remove them to not immediately break older nightlies.
 // When all features are stable, we'll remove them.
 #![allow(stable_features)]
+#![allow(unknown_lints)]
+#![allow(async_fn_in_trait)]
 #![feature(async_fn_in_trait, impl_trait_projections)]
 
 pub mod delay;

embedded-io-async/src/lib.rs

@@ -7,6 +7,8 @@
 // We don't immediately remove them to not immediately break older nightlies.
 // When all features are stable, we'll remove them.
 #![allow(stable_features)]
+#![allow(unknown_lints)]
+#![allow(async_fn_in_trait)]
 #![feature(async_fn_in_trait, impl_trait_projections)]
 
 #[cfg(feature = "alloc")]