RUST: Hide the C types behind a feature to forbid any usage by safe creates

src/rust/liblektor-rs/lektor_c_compat/Cargo.toml

src/rust/liblektor-rs/lektor_c_compat/src/lib.rs

src/rust/liblektor-rs/lektor_repo/src/lib.rs

src/rust/liblektor-rs/lektor_unsafe/Cargo.toml

src/rust/liblektor-rs/lektor_unsafe/src/lib.rs

src/rust/liblektor-rs/lektor_unsafe/src/repo.rs