Consider redesigning AggregateError

[yoshuawuyts]

RaceOk returns a Result<T, AggregateError> where the error type matches the input type. Array of futures in -> array of futures out. etc.

Right now the API for it is rather clumsy; usually a thin wrapper to the underlying type + an Error impl so you can pass it through as-is. But what we really want is to expose something akin to Error::sources but not to iterate down, but to iterate over the siblings contained within the aggregate. Not entirely sure yet how to best approach this though.