Optimize check for key existence

[declanvk]

Currently the contains_key method is very simple:

    pub fn contains_key<K>(&self, k: K) -> bool
    where
        K: AsRef<[u8]>,
    {
        self.get(k).is_some()
    }

Possibly we could optimize this so that it doesn't return any extra information, but just fast returns if the key is present or missing. Needs specific benchmark results