Rust frontend emits deprecated clippy suppression attributes

Currently, following lines are inserted:

#![cfg_attr(feature = "cargo-clippy", allow(too_many_arguments, type_complexity))]

But this syntax is deprecated now.

Following should be used instead:

#![allow(clippy::too_many_arguments, clippy::type_complexity)]

Note: I fixed this issue in my fork, but I'm not sure if it will be easier for developers to merge my PR than apply changes themselves. Also, I'm student and my Git skills are not very good)