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