Address lints from clippy for beta/nightly (#5709)

* Fix clippy lints in tests

* Replace `format!` in `.push_str()` with `write!`

Stylistically that might be a bit rough but elides an allocation.

Fallibility of allocation is more explicit, but ignored with `let _ =`
like in the clippy example:

https://rust-lang.github.io/rust-clippy/master/index.html#format_push_string

* Remove unused lifetime

* Fix macro crate relative import

* Derive `Eq` for `PartialEq` with `Eq` members

https://rust-lang.github.io/rust-clippy/master/index.html#derive_partial_eq_without_eq

* Remove unnnecessary `.to_string()` for Cow<str>

* Remove `.to_string()` for `tendril::Tendril`

Implements `Deref<Target = str>`

crates/nu-command/src/system/exec.rs

@@ -87,7 +87,7 @@ fn exec(
         redirect_stderr: false,
     };
 
-    let mut command = external_command.spawn_simple_command(&cwd.to_string_lossy().to_string())?;
+    let mut command = external_command.spawn_simple_command(&cwd.to_string_lossy())?;
     command.current_dir(current_dir);
 
     let err = command.exec(); // this replaces our process, should not return