D6593: rust-minor-fixes: remove Deref in favor of explicit methods

[Yuya Nishihara]

> +    pub fn contains_key(&self, key: &[u8]) -> bool {
> +        self.inner.contains_key(key)
> +    }
> +
> +    pub fn iter(&self) -> Iter<Vec<u8>, u32> {
> +        self.inner.iter()
> +    }

Again,

- **contains**() -> inner.contains_key()
- iter() -> inner.**keys**()

Somewhat similar to HashSet, which is basically a proxy type to
HashMap<T, ()>.

I queued this as I think it's strictly better than using Deref, but I expect
a follow up patch. Thanks.