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-plugin/src/protocol/mod.rs

@@ -18,7 +18,7 @@ pub enum PluginCall {
     CallInfo(Box<CallInfo>),
 }
 
-#[derive(Clone, PartialEq, Serialize, Deserialize, Debug)]
+#[derive(Clone, PartialEq, Eq, Serialize, Deserialize, Debug)]
 pub struct LabeledError {
     pub label: String,
     pub msg: String,