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/viewers/icons.rs

@@ -428,7 +428,7 @@ pub fn extension_is_one_of(path: &Path, choices: &[&str]) -> bool {
 //     choices.contains(&&name[..])
 // }
 
-#[derive(Debug, Default, PartialEq)]
+#[derive(Debug, Default, PartialEq, Eq)]
 pub struct FileExtensions;
 
 // TODO: We may want to re-add these FileExtensions impl fns back. I have disabled