Implement Storable for Vec<T> where T: Storable

[ielashi]

Currently we only implement Storable for Vec<u8>. Ideally, we would have a generic implementation that allows developers to put anything that implements Storable in a Vec.

We can't introduce this implementation at this point because:

• Vec<u8> is special, in the sense that we don’t need to do any serializing/deserializing for the data - which makes its Storable implementation fast.
• For all other types, we’d need to serialize/deserialize the elements in the Vec.

We'd need to wait for generic specializations in Rust to encode the logic above. The Vec<u8> special-case is quite important as it’s very often used.