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-json/src/error.rs

@@ -13,7 +13,7 @@ use serde::de;
 use serde::ser;
 
 /// The errors that can arise while parsing a JSON stream.
-#[derive(Clone, PartialEq)]
+#[derive(Clone, PartialEq, Eq)]
 pub enum ErrorCode {
     /// Catchall for syntax error messages
     Custom(String),