Fix Trac #959: a long-standing bug in instantiating otherwise-unbound type variables

Fix Trac #959: a long-standing bug in instantiating otherwise-unbound type variables

   DO NOT MERGE TO GHC 6.12 branch
   (Reason: interface file format change.)

The typechecker needs to instantiate otherwise-unconstraint type variables to
an appropriately-kinded constant type, but we didn't have a supply of
arbitrarily-kinded tycons for this purpose.  Now we do.

The details are described in Note [Any types] in TysPrim.  The
fundamental change is that there is a new sort of TyCon, namely
AnyTyCon, defined in TyCon.

Ter's a small change to interface-file binary format, because the new
AnyTyCons have to be serialised.

I tided up the handling of uniques a bit too, so that mkUnique is not
exported, so that we can see all the different name spaces in one module.