Remove unused stream id to definition id mapping
Clean up the read core functionality

It was a little bit expansive, mostly due to the implementation also
being part discovery process (of the argot format).

The core body verification functionality is rightfully the
responsibility of the Library class and so, has also been moved into
that same class.
Stop calling callbacks with promises

The load dictionary and read message functions were both accepting
callbacks and then calling them with promise values, an unessessary
duplication of effort and source of confusion.

This change removes the callbacks and has the function each just return
a promise. I was unsure if this route was acceptable previously because
the nessessary path to accomplish this appeared to result with promises
nested inside promises but, fortunately, q handles all of this with ease
and does not nest.