This massive commit is what Simon has been up to for a couple of weeks.

1.  Scoped type variables are in

2.  The typechecker works a bit differently.
    In partiular, the compiler no longer has TcTyVars of
	a different type than TyVars.
    All the 's' and 'flexi' type parameters have vanished from Id, TyVar,
	Type, etc.
    The typchecker monad is now in the IO world (though I didn't get
	around to removing the 's' parameter from the monad, but it's
	no longer used)

    Bottom line: significantly simpler,
		 fewer gratuitous conversions from TcType <-> Type
		 but less type security in the compiler

    There was a reason for doing this now; somehow the 's' stuff
    got in the way of kind inference for scoped type variables
    and I lost patience with it.

3.  Haskell98-style reporting of scope errors; i.e. you only get
    an error if you use a variable that could mean two different things.
    At the same time I did a lot of tidying-up in the renamer.

4.  Mostly-complete fix to the reporting of unused variables, which
    has never worked properly.  (The 'mostly' bit is because it reports
    those 'system' tycons like _C as unused.  I'm on the job.)

5.  The parser is a bit tider than it was.  A few more ugn files give
    a more refined C data type.  I had to tackle this because of
    the scoped type variables.

6.  Haskell98-style fixities.  Fixity decls can occur wherever a type
    signature can

7.  Some HsSyn changes that constitute minor tidy ups
	Put TypeDecl and ClassDecl into one type [HsDecls]
	Improved the HsMatch/GRHSs etc data types.

8.  TcGRHSs is removed; combined into TcMatches.

I DO NOT PROMISE THAT ALL OF THIS WORKS.  It compiles the Prelude,
but I have not tested it more than that.  Stick to 4.01 if you want a
compiler that's sure to work.