Very simple, backward-compatible change.  Not sure how this never
happened before, it would be a shame to have worked on the
preprocessor interface and not exposed it to users at all.

Took this chance to move the test preprocessor out of the
PreProcessors module and into a test case.

User hooks look like this now:
     preConf  :: Args -> ConfigFlags -> IO HookedBuildInfo,
     postConf :: IO ExitCode,

type HookedBuildInfo = (Maybe BuildInfo, [(String, BuildInfo)])

this means that only stuff in BuildInfo can be overridden.  A
buildinfo file now looks like this:

name: libraryname
include-dirs: /foo/bar, /bang/baz
extensions: CPP

executable: executablename
extensions: TemplateHaskell

if ./setup haddock is run w/ verbosity > 4, it adds -v to haddock command

Introduced new data type, Library which is symmetric to Executable.
Removed "executable only" and "library only" fields from
PackageDescription and moved them to their respective internal types.

Moved Build-depends out of buildinfo and into the package description
proper, as per Ross's suggestion.

Cleaned up parser and test cases accordingly.

Now it searhes for *.cabal in the current directory, if it finds more
than one, there's an error.  For now, it allows Setup.description with
a warning.

** Previously, "hidden-modules" had nothing to do with what gets
   built, and we had to compute "exposed-modules" with "modules `setdiff`
   hidden modules".  At Ross Paterson's suggestion, I added
   Exposed-Modules.

** Since the executable sections also used the "modules" field, I
   changed it to use "executable-modules", and got rid of the
   ambiguous "modules" field.  This should hopefully be more clear to
   everyone.

End-to-end preprocessing solution, including some test cases.
One test case fails:
  right now, preprocessors just stick files into the current
  directory, then copies them into the destination.  this is no good
  since we now won't be able to tell them from real source files the
  next time around.  clean is broken here...

** removed hugs test case that never works
** added -DDEBUG flags to HUnit and A
** fixed A's description file up a bit
** tried to cut back on cascading failures in moduleTest