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-parser/tests/test_parser.rs

@@ -156,10 +156,7 @@ pub fn parse_binary_with_invalid_octal_format() {
     assert!(block.len() == 1);
     let expressions = &block[0];
     assert!(expressions.len() == 1);
-    assert!(match &expressions[0].expr {
-        Expr::Binary(_) => false,
-        _ => true,
-    })
+    assert!(!matches!(&expressions[0].expr, Expr::Binary(_)))
 }
 
 #[test]