Check in the beginnings of a SimpleBackend instance for C.hs
Switch the C backend over to a SimpleBackend-based run.  Passes 63/64 tests on mac os (same as usual).
C.hs: Fill out some more of the Backend instances
Use ifdef level hacks to share Cilk.hs and C.hs
Add beginnings of a library for dynamically constructing full Acc programs.
Some progress on DynamicAcc