[gen.rs] Improve `cenum_to_rs`.
[gen.rs] Improve `cenum_to_rs`.

Twinklebear/rust-bindgen,alexander255/rust-bindgen,Twinklebear/rust-bindgen,bsteinb/rust-bindgen,bsteinb/rust-bindgen,Twinklebear/rust-bindgen,Twinklebear/rust-bindgen,bsteinb/rust-bindgen,alexander255/rust-bindgen,alexander255/rust-bindgen