stm32: add rust stable support

embassy-stm32/src/lib.rs

@@ -1,6 +1,8 @@
 #![no_std]
-#![feature(generic_associated_types)]
-#![feature(type_alias_impl_trait)]
+#![cfg_attr(
+    feature = "nightly",
+    feature(generic_associated_types, type_alias_impl_trait)
+)]
 
 #[cfg(feature = "unstable-pac")]
 pub use stm32_metapac as pac;