Reduce length-prepending in BFieldCodec encoding through the use of Cursor

[Sword-Smith]

By using a Cursor data struct, I think we can avoid the prepending of the struct length when encoding both Vec<T> and [T; N]. Each element is double length prepended since the length prepending happens in both the encoding implementation for Vec<T> and T when the encoding length is not known at compile time.

I think this can be avoided if we use a type of Cursor<Vec<BFieldElement>> for the decoding process.