Revisit structure at runtime of allOf/oneOf/anyOf

Structure at runtime of allOf/oneOf/anyOf - today, we use nesting of structs to represent the nested schemas.

Could be represented as a single flattened struct that conforms to multiple protocols, one for each nested type, or some other way.

Things to keep in mind:
- lack of nested protocols in Swift
- cost of using existentials vs concrete types

Review the current runtime representation of nested allOf, oneOf, and anyOf schemas. Compare the proposed flattened-struct and nested-struct approaches in light of Swift's lack of nested protocols and the cost of existentials versus concrete types. Done means selecting and documenting a representation with its trade-offs.

Written by the indexing model from the issue text.