#1999 let a record channel keep its own type beside a dyn one, for the
yields of an IterableIterator<Row>. It applied to the return channel too,
so a generator typed with a dyn yield and a record return converted its
return value into that record. Effect.gen's generators are typed that way:
`yield* RpcTest.makeClient(group)` answered the client converted to a
record without its methods, and `client.double(...)` threw. Only a record
yield channel now keeps its type beside a dyn return.
Chris Tate
fd3155fc39