tcplib should use iovec writes for the __DoSendMsg inside of SendMsg, which
could potentially double the efficiency of the call.
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Design issue of callbacks.
What I want is the typechecked first class functions that you get as 
part of SML. e.g. the function can keep around it's own data without
me having to pass it, and I can send all of the arguments to the
callback in a "normal" way.  How should this be implemented in C?
