Clippy lints update: all is implied, add todo

I must have been mistaken when originally introducing `clippy:all`, because these are, by definition, lints that are enabled without any configuration: https://doc.rust-lang.org/stable/clippy/usage.html#lint-configuration

Add a lint that catches `todo!()` (but allows `panic!()`, `unreachable!()`, `unimplemented!()`) - you're encouraged to use it during development as a reminder for stuff to finish before a change can be merged.

src/lib.rs

@@ -1,4 +1,4 @@
-#![warn(clippy::all, clippy::clone_on_ref_ptr)]
+#![warn(clippy::clone_on_ref_ptr, clippy::todo)]
 
 //! This crate aims to provide a minimalist and high-performance actor framework
 //! for Rust with significantly less complexity than other frameworks like