Pipe send should return whether the receiver might receive the packet

[eholk]

Pipes can detect if the other end is closed. Receiving from a closed pipe reports this to the caller, but send does not. This is easy enough to add, and we should.

[eholk]

So, send currently returns the next state of the protocol. I'm fixing this by instead generating try_ versions of all the send functions, which return an option(next_state), which is analogous to how the try_recv functions work.