Rework ast::BinOpKind::to_string and ast::UnOp::to_string.

- Rename them both `as_str`, which is the typical name for a function
  that returns a `&str`. (`to_string` is appropriate for functions
  returning `String` or maybe `Cow<'a, str>`.)
- Change `UnOp::as_str` from an associated function (weird!) to a
  method.
- Avoid needless `self` dereferences.

clippy_utils/src/sugg.rs

@@ -382,7 +382,7 @@ fn binop_to_string(op: AssocOp, lhs: &str, rhs: &str) -> String {
         | AssocOp::GreaterEqual => {
             format!(
                 "{lhs} {} {rhs}",
-                op.to_ast_binop().expect("Those are AST ops").to_string()
+                op.to_ast_binop().expect("Those are AST ops").as_str()
             )
         },
         AssocOp::Assign => format!("{lhs} = {rhs}"),