Stage: Candidate
Type: Union[type_0, type_1, ...]
Default: default(type_0)
A Union provides the ability to represent a set of predetermined types in the same tree and serialization position.
A special null type may be used as first type parameter to emulate an Option, any other type parameter than the first MUST not be null.
A null as a standalone type is illegal.
An Union is considered to have a dynamic encoding-size, even if all the selectable options have the same type or happen to have the same serialized byte length.
Serialization is defined as an uint32 for the type index, followed by the serialization of the selected option.
null is represented as an empty byte sequence (i.e. remaining scope after the type_index is 0).
Merkleization is defined as mix_in_num(x, i) where x is the root of the selected option with index i (right-padded to 32 bytes, effectively an uint256).