Added GHC formalism to the GHC source tree.
As per a request from Simon PJ, I wrote up a formalism of the core language in GHC, System FC. The writeup lives in docs/core-spec. I also added comments to a number of files dealing with the core language reminding authors to update the formalism when updating the code. In the next commit will be a README file in docs/core-spec with more details of how to do this.