Implementations - ManagedVec - Rust + Mx

[virgil-serbanuta]

Needed for #34

Needs hook-based k-based representation with implementations for

• ::new()
• push(_)
• get(_)
• set(_, _)
• len()
• into() that returns MultiValueEncoded. We probably want a hack that just returns the MultiValueEncoded, not a full into implementation that selects the right kind of value to return and so on. It's probably better if MultiValueEncoded is just a wrapper around ManagedVec.