Allow unused crates in Rust files

Currently thrift generated .rs files include following attributes:

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

I propose to add
#![allow(unused_extern_crates)]

to remove warning when using Rust nightly.