Refactor Continuations

Coninuations have been refactored to eliminate the ContM Monad. A
semi-stable interface has been established.  This commit does not include
the work necessary to impliment Object and Eval using this interface.

Some minimal testing and benchamrking has been done.  Initial results
indicate that `dispatch` is approximatly an order of magnitude slower then
send.  Interstingly, with this overly simplified benchmark, the single
threaded application is significantly faster then the multi threaded one.
I am suspicious that in real world applications the threaded
implimentation will inprove and the overhead of `dispatch` will be
minimal.