lints: Warn rather than deny by default

We check with `-Dwarnings` in CI, so these still get checked. This
change serves to prevent failures to compile downstream if the lint
expectations are not met on untested targets.

(backport <#4473>)
(cherry picked from commit 260e357)

Author: tgross35

src/lib.rs

@@ -25,8 +25,8 @@
 // DIFF(1.0): The thread local references that raise this lint were removed in 1.0
 #![cfg_attr(feature = "rustc-dep-of-std", allow(static_mut_refs))]
 // Enable extra lints:
-#![cfg_attr(feature = "extra_traits", deny(missing_debug_implementations))]
-#![deny(missing_copy_implementations, safe_packed_borrows)]
+#![cfg_attr(feature = "extra_traits", warn(missing_debug_implementations))]
+#![warn(missing_copy_implementations, safe_packed_borrows)]
 #![cfg_attr(not(feature = "rustc-dep-of-std"), no_std)]
 #![cfg_attr(feature = "rustc-dep-of-std", no_core)]