Add Item associated type to VectorCommitment

[Al-Kindi-0]

Investigate adding Item associated type and consider adding a trait bound on it to build items from lists of field elements - e.g., something like:

type Item: Serializable + Deserializable + From<&[Self::FieldElement]>;

This would also mean that we'd need to add FieldElement associated type to the trait.

Originally posted by @irakliyk in #285 (comment)