Cabal suggestions

[Ross Paterson]

* How about replacing the modules field with hidden-modules, to avoid
  repetition?

* setup install and register have --user and --system flags, which seem
  to offer a choice at that point, but one has to have anticipated this
  choice when supplying the --prefix flag to configure.  How about a
  --user-prefix flag to configure (with a sensible default) to say where
  user packages are installed?

* It would be useful to have alternatives in dependencies, e.g. HGL
  could depend on X11 | Win32.

* The compilerBinaryName for Hugs should be "ffihugs" (which compiles
  FFI stubs).

* moveSources seems misnamed, because it copies rather than moving, and
  operates on the relult of building as well as sources.

* Could singleStanza ignore comments like splitStanzas does?

* How about using a new data type for parse results instead of Either?
  That would remove the dependency on mtl, which will help with bootstrapping.
  I know there is Distribution.Compat.Error, but that defines a Monad
  instance for Either, so it one uses that there'll be trouble later if
  some program includes both Cabal and mtl.