The main body for a handler ...
The main body for a handler ... registers a function ('onReq') which does some work.
Instead of a thinking of a generic computation as a function from A => B, This exposes the function as WorkContext[A] => ResponseEntity
The WorkContext exposes a handle onto the exchange (so the computation can request more work) and access details about the work sent to it
the request input type
the compute functions
the subscription to use when asking for work for this computation
how many work items to initially ask for
a future of the 'request work' ack