macro or inspect operator for debugging that tees any variable to stdio

[jhellerstein]

Would be nice for debugging to have an easy syntax to mark a hydroflow variable "debug" and have its contents teed to stdio or stderr.

I.e. suppose I have

message_generator = recv_iter([1,2,3]) -> ... foo(...) -> sink_async(..);

I would want:

message_generator_prep = recv_iter([1,2,3]) -> ... foo(...) -> tee();
message_generator_prep[0] -> foreach(|m| println!(message_generator: {"?"}, m);
message_generator = message_generator_prep[1] -> sink_async(..);