This needs to be implemented as it's own class instead of using
one of the helpers like `fromHandlers` because it has unique generic
type requirements. We want to avoid requring the call site to re-specify
the generic type of the input stream. The return type needs to be a
`StreamTransformer<Null, R>` to allow to satisfy static checking.
However the argument to `bind` at runtime will never be a `Stream<Null>`
so we need to define it explicitly to widen the allowed argument.