Add a compileToCore function to the GHC API

Added a compileToCore function to the GHC API that takes a
  session, module, and filename, and returns a list of Core
  bindings if successful. This is just a first try and could
  probably be improved (for example, there's probably a way to
  get the filename from the module so that it doesn't have to
  be passed in, I just don't see it offhand.)