rust: do a clippy pass This is the result of running `cargo clippy` on hg-core/hg-cpython and fixing the lints that do not require too much code churn (and would warrant a separate commit/complete refactor) and only come from our code (a lot of warnings in hg-cpython come from `rust-cpython`). Most of those were good lints, two of them was the linter not being smart enough (or compiler to get up to `clippy`'s level depending on how you see it). Maybe in the future we could have `clippy` be part of the CI. Differential Revision: https://phab.mercurial-scm.org/D8635

--- a/rust/hg-cpython/src/dirstate/non_normal_entries.rs	Mon Jun 15 15:14:16 2020 -0400
+++ b/rust/hg-cpython/src/dirstate/non_normal_entries.rs	Mon Jun 15 18:26:40 2020 +0200
@@ -62,7 +62,7 @@
         py: Python,
         key: &HgPathBuf,
     ) -> PyResult<Option<PyBytes>> {
-        Ok(Some(PyBytes::new(py, key.as_ref())))
+        Ok(Some(PyBytes::new(py, key.as_bytes())))
     }
 }