No change in functionality here, but greater clarity:

* In FamInstEnv.FlattenEnv, kill off the fi_in_scope field
  We are already maintaining an in-scope set in the fe_subst field,
  so it's silly do to it twice.

  (This isn't strictly connected to the rest of this patch, but
  the nomenclature changes below affect the same code, so I put
  them together.)

* TyCoRep.extendTCVSubst used to take a TyVar or a CoVar and work
  out what to do, but in fact we almost always know which of the
  two we are doing.  So:
    - define extendTvSubst, extendCvSubst
    - and use them

* Similar renamings in TyCoRep:
   - extendTCvSubstList        -->   extendTvSubstList
   - extendTCvSubstBinder      -->   extendTvSubstBinder
   - extendTCvSubstAndInScope  --> extendTvSubstAndInScope

* Add Type.extendTvSubstWithClone, extendCvSubstWithClone

* Similar nomenclature changes in Subst, SimplEnv, Specialise

* Kill off TyCoRep.substTelescope (never used)