This commit makes a start at implementing polymorphic usage
annotations.

* The module Type has now been split into TypeRep, containing the
  representation Type(..) and other information for `friends' only,
  and Type, providing the public interface to Type.  Due to a bug in
  the interface-file slurping prior to ghc-4.04, {-# SOURCE #-}
  dependencies must unfortunately still refer to TypeRep even though
  they are not friends.

* Unfoldings in interface files now print as __U instead of __u.
  UpdateInfo now prints as __UA instead of __U.

* A new sort of variables, UVar, in their own namespace, uvName, has
  been introduced for usage variables.

* Usage binders __fuall uv have been introduced.  Usage annotations
  are now __u - ty (used once), __u ! ty (used possibly many times),
  __u uv ty (used uv times), where uv is a UVar.  __o and __m have
  gone.  All this still lives only in a TyNote, *for now* (but not for
  much longer).

* Variance calculation for TyCons has moved from
  typecheck/TcTyClsDecls to types/Variance.

* Usage annotation and inference are now done together in a single
  pass.  Provision has been made for inferring polymorphic usage
  annotations (with __fuall) but this has not yet been implemented.
  Watch this space!