Create a new DS called `Fluid dictionary`

This should be the polymorphic brother of the standard dictionary.

This is the interface we should implement:
```C#
public interface IFluidDictionary<TKey>
{
    bool Set<T>(TKey key, T value);
    bool TryGetValue<T>(TKey key, out T value);
    T GetValueOrDefault<T>(TKey key);
    T GetRequiredValue<T>(TKey key);
    IEnumerable<TKey> Keys { get; }
}
```