RUST: Add wrappers for queue functions to call rust stuff from C

inc/liblektor-rs/database.h

inc/liblektor-rs/queue.h

src/rust/lektor_db/src/queue.rs

src/rust/lektor_unsafe/src/db.rs

src/rust/lektor_unsafe/src/lib.rs

src/rust/lektor_unsafe/src/queue.rs

src/rust/lektor_unsafe/src/repo.rs