Nest helper types to tidy up top-level namespace

[pcantrell]

Swift now allows generic types to have nested types. Several enums ended up at the top level of Siesta’s namespace only because the type they would ideally be nested inside didn’t previously allow that nesting:

• InputTypeMismatchAction should be a member of ResponseContentTransformer.
• StandardTransformer probably should be too.

There may now be other types types where nesting would make sense. Explore those. (Ensure that deprecated typealias fix backward compatibility.)

Protocols still disallow most nested types, so some things will unfortunately have to remain at the top level:

• ResourceEvent can’t be ResourceObserver.Event.
• RequestChainAction can’t be Request.ChainAction.