This lets us gain coverage on programs that use fork, like xmonad.

* To be merged to STABLE *

* To be merged to STABLE. *

This change allows you to write
	f :: (Eq a) => (Ord b) => a -> b -> b
Previously you could only have a forall and context after '->'
but not after '=>' which is strange and inconsistent.

Making the parser a bit more generous didn't change the number
of shift/reduce conflicts.

tc236 tests.

- checkTauTvUpdate now distinguishes between whether
  (1) a type variables occurs only in type family parameters
      (in which case unification is to be deferred)
  (2) other variable occurences
      (which case we fail with a cannot create infinite type message, as before)

Fill all of the sigevent structure with zeroes before individual fields are
set. Although not strictly necessary, this keeps tools like valgrind from
complaining about passing uninitialized data, which is a good thing.

MERGE TO STABLE

installPackage is a horror, using --force is probably not the perfect way
to make DESTDIR functionality work again, but given the current state of
affairs, I can't find a quick and clean solution.

MERGE TO STABLE

Having all package directory directly in the toplevel installation directory
would be a bad thing because of potential name clashes, and it wasn't done this
way earlier, so reinstantiate the old layout.

MERGE TO STABLE

- Cleaned up and simplified rules
- Added many missing cases
- The rules OccursCheck and swap have been eliminated and integrate with
  the other rules; ie, Subst and Unify perform the occurs check themselves
  and they can deal with left-to-right and right-to-left oriented rewrites.
  This makes the code simpler and more efficient.
- Also added comments.

Watch out: I've added MkZipCfg.sequence, which clashes with the
Prelude (like some other names in MkZipCfg.  Maybe you can think
of a better name for it.

When trying to do it with -D we were having trouble getting the double
quotes through in all Windows configurations.