First cut at reviving the External Core tools

First cut at reviving the External Core tools

I updated the External Core AST to be somewhat closer to reality (where reality is defined by the HEAD), and got all the code to compile under GHC 6.8.1. (That means it works, right?)

Major changes:

- Added a Makefile.

- Core AST:
    - Represented package names and qualified module names.
    - Added type annotation on Case exps.
    - Changed Coerce to Cast.
    - Cleaned up representation of qualified/unqualified names.
    - Fixed up wired-in module names (no more "PrelGHC", etc.)

- Updated parser/interpreter/typechecker/prep for the new AST.

- Typechecker:
    - Used a Reader monad to pass around the global environment and top module name.
    - Added an entry point to check a single expression.

- Prep:
    - Got rid of typeofExp; it's now defined in terms of the typechecker.