rust-clippy: disable some lints crate-wide for `hg-cpython`

`rust-cpython` creates some pretty funky code that also needs to be compatible
with pretty old Rust. This makes clippy quite useless in `hg-cpython` unless
you disable the lints that are always triggered by `py_class!` and related.

Maybe `clippy` will allow one day to exclude a dependency from its linting,
but this seems quite unlikely, so this is the best we've got at the moment.

rust/hg-cpython/src/lib.rs

@@ -18,6 +18,11 b''
 //! >>> ancestor.__doc__
 //! 'Generic DAG ancestor algorithms - Rust implementation'
 //! ```
+#![allow(clippy::too_many_arguments)] // rust-cpython macros
+#![allow(clippy::zero_ptr)] // rust-cpython macros
+#![allow(clippy::needless_update)] // rust-cpython macros
+#![allow(clippy::manual_strip)] // rust-cpython macros
+#![allow(clippy::type_complexity)] // rust-cpython macros
 
 /// This crate uses nested private macros, `extern crate` is still needed in
 /// 2018 edition.