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-engine/src/eval.rs

@@ -1108,7 +1108,7 @@ pub fn create_scope(
                 if !alias_text.is_empty() {
                     alias_text.push(' ');
                 }
-                alias_text.push_str(&String::from_utf8_lossy(contents).to_string());
+                alias_text.push_str(&String::from_utf8_lossy(contents));
             }
             aliases.push((
                 Value::String {