Test FFI safety

Given the name of the crate I would expect to have something to verify that the generated data structure is considered FFI-safe by rustc. I've seen rustc periodically introduce regressions in what is considered to be FFI-safe.

Something like:

```rust
#[deny(improper_ctypes, warnings)]
extern "C" {
    pub fn f(_: *const leveldb_t);
}
```