net: feature-gate nightly-only async traits to allow building on stable.

embassy-net/src/lib.rs

@@ -1,6 +1,5 @@
 #![cfg_attr(not(feature = "std"), no_std)]
-#![allow(clippy::new_without_default)]
-#![feature(generic_associated_types, type_alias_impl_trait)]
+#![cfg_attr(feature = "nightly", feature(generic_associated_types, type_alias_impl_trait))]
 
 // This mod MUST go first, so that the others see its macros.
 pub(crate) mod fmt;