Remove private_in_public lint (#128)

### Checklist

* [x] I have read the [Contributor Guide](../../CONTRIBUTING.md)
* [x] I have read and agree to the [Code of
Conduct](../../CODE_OF_CONDUCT.md)
* [x] I have added a description of my changes and why I'd like them
included in the section below

### Description of Changes

Remove `private_in_public` which causes a CI build failure:
https://rust-lang.github.io/rfcs/2145-type-privacy.html

Author: fornwall

crates/mirror-mirror-macros/src/lib.rs

@@ -33,7 +33,7 @@
     missing_debug_implementations,
     // missing_docs
 )]
-#![deny(unreachable_pub, private_in_public)]
+#![deny(unreachable_pub)]
 #![allow(elided_lifetimes_in_paths, clippy::type_complexity)]
 #![forbid(unsafe_code)]
 #![cfg_attr(docsrs, feature(doc_auto_cfg, doc_cfg))]

crates/mirror-mirror/src/lib.rs

@@ -245,7 +245,7 @@
     missing_debug_implementations,
     // missing_docs
 )]
-#![deny(unreachable_pub, private_in_public)]
+#![deny(unreachable_pub)]
 #![allow(
     elided_lifetimes_in_paths,
     clippy::type_complexity,