A set of example programs for handling external core format.

In particular, typechecker and interpreter give a precise semantics.
---------------------
tjc April/May 2008:

==== Documentation ====

Documentation for the External Core format lives under docs/ext-core in the
GHC tree. If you are building from HEAD, do not rely on the version of the
External Core documentation that lives in haskell.org -- it is obsolete!

==== Notes ====

The checker should work on most programs. Bugs (and infelicities) 
I'm aware of:
   
1. There's some weirdness involving funny character literals. This can
   be fixed by writing a new lexer for chars rather than using Parsec's
   built-in charLiteral lexer. But I haven't done that.

2. The test driver attempts to find module dependencies automatically,
   but it's slow. You can turn it off with the "-n" flag to the driver,
   and specify all dependencies on the command line (you have to include
   standard library dependencies too.)
     * It would help to cache dependency info for standard libraries
       in a file, or something, but that's future work.

3. To avoid implementing some of the I/O primitives and foreign calls,
   I use a gross hack involving replacing certain standard library
   modules with simplified versions (found under lib/) that depend on
   fake "primops" that the Core interpreter implements. This makes it
   difficult (if not impossible) to load optimized versions of standard
   libraries right now. Fixing this is future work too.

Typechecking all the GHC libraries eats about a gig of heap and takes a
long time. I blame Parsec. (Someone who was bored, or understood happy
better than I do, could update the old happy parser, which is still in the
repo.)

The interpreter is also memory-hungry, but works for small programs
that only do simple I/O (e.g., putStrLn is okay; not much more than that)
and don't use Doubles or arrays. For example: exp3_8, gen_regexps, queens,
primes, rfib, tak, wheel-sieve1, and wheel-sieve2, if modified so as not
to take input or arguments.

==== Building ====

To run the checker and interpreter, you need to generate External Core
for all the base, integer and ghc-prim libraries. This can be done by
adding "-fext-core" to the GhcLibHcOpts in your build.mk file, then
running "make" under libraries/.

Then you need to edit Driver.hs and change "baseDir" to point to your GHC
libraries directory.

Once you've done that:
1. make prims (to generate the primops file)
2. make
3. make nofibtest (to run the parser/checker on all nofib programs...
   for example.)

Tested with GHC 6.8.2. I make no claims of portability.