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.