Make backing binary tree structure 100% static

[Nashatyrev]

Creating this issue just to discuss the following idea:

currently the backing tree structure is not static due to List and Union types. E.g. when an element is appended to a non-basic type List the tree structure changes: the Zero leaf is replaced by a new element's subtree. The similar applies to the Union type.

What if make the default List[T, N] backing tree the same way as with Vector[T, N]? I.e. tree nodes for non-existing list elements would be the default tree of type T instead of Zero leaf. With this approach the List backing tree would have the same structure independently of content it's representing.

With the above approach the List[T, N] could be represented exactly as Container(data: Vector[T, N], length: UInt64) and would have corresponding merkleization

Following the similar idea the Union[T1, ..., Tn] could be represented as Container(t1: T1, ..., tn: Tn, typeIndex: UInt64)

I believe making the backing binary tree structure static could make the whole SSZ concept a bit clearer and solid, enable more clear implementations and better optimizations.

This idea is non backward compatible with merkleization so could be potentially considered for say SSZ v2