[gen.rs] Improve cfuncty_to_rs.
[gen.rs] Improve cfuncty_to_rs.

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