A CompletedRequestMap is a universally quantified mapping from requests of
type Request[E, A] to results of type Either[E, A] for all types E and
A. The guarantee is that for any request of type Request[E, A], if there
is a corresponding value in the map, that value is of type Either[E, A].
This is used by the library to support data sources that return different
result types for different requests while guaranteeing that results will be
of the type requested.