zeroize_derive: Deprecate #[zeroize(no_drop)] attribute

The `no_drop` attribute existed because the initial proc macro
implicitly added a `Drop` impl unless instructed otherwise.

It turns out there are several legitimate reasons not to derive `Drop`,
so this was reversed in v0.9, with `drop` or `no_drop` attributes
mandatory to ensure everyone migrated and was not expecting an implicit
`Drop` handler to be derived (this was further enforced by yanking v0.8
which was the only version with this behavior).

Now that everyone has migrated, `no_drop` no longer serves any purpose,
so this deprecates it by printing a warning.

Author: tony-iqlusion

zeroize_derive/src/lib.rs

@@ -77,7 +77,10 @@ impl DeriveAttrs {
         if ident == "drop" {
             self.set_drop_flag(true);
         } else if ident == "no_drop" {
-            self.set_drop_flag(false);
+            eprintln!(
+                "warning: use of deprecated attribute #[zeroize(no_drop)]: \
+                 has no effect and will be removed in zeroize 1.0"
+            );
         } else {
             panic!("unknown #[zeroize] attribute type: {}", ident);
         }