Expose Key trait as public to allow custom data types as leveldb keys

[cassc]

Currently there is no way to use our own data types as keys because the module is private, for example this will not work

// ...
use leveldb::database::key::Key; // error[E0603]: crate `key` is private

#[derive(Clone)]
struct MyData {
    key: String,
}

impl Key for MyData {
    fn from_u8(key: &[u8]) -> Self {
        MyData {
            key: str::from_utf8(key).unwrap().into(),
        }
    }

    fn as_slice<T, F: Fn(&[u8]) -> T>(&self, f: F) -> T {
        let dst = self.key.as_bytes();
        f(&dst)
    }
}
// ...

This PR creates a simple fix by removing the db-key dependency and adds the Key trait as public in src/database/key.rs.

A full example is here

[kaimast]

I saw this after commenting on #52. It really seems overly complicated to have db-key as a separate crate, unless other k/v stores are using it, but I do not think they are.

The PR works fine for me. All tests pass. I see some warnings, but those are on master too.