interim commit: wrote Receiver but got side-tracked

The issue is that wrapping is getting in the way all over the place, it
is fragile and ugly. The only reason for it to exist is to transport a
failure response out of the handling of a Failed message, and that can
more reliably be done in a side-effecting fashion (like for
setReceiveTimeout). So, I’ll just stash this work and rip out Wrapper.
removed Behavior.Wrapper

The response to Failed signals is now communicated out of band by
calling ctx.setFailureResponse. No type-safety is lost since the
previous approach also did not enforce that the behavior react to the
signal with a Wrapper (as opposed to just returning a naked Behavior).
add LineNumbers support