* Comments to explain that a CoVar, whose IdInfo is CoVarId,
  is always unlifted (but may be nominal or representational role)

  And TyCoRep.isCoercionType picks out only those unlifted
  types, NOT the lifted versions

* Introduce Var.NcId for non-co-var Ids
  with predicate isNonCoVarId

* Add assertions in CoreSubst that the Id env is only
  used for NcIds

* Fix lurking bug in CSE which extended the
  CoreSubst Id env with a CoVar

* Fix two bugs in Specialise.spec_call, which wrongly treated
  CoVars like NcIds
    - needed a varToCoreExpr in one place
    - needed extendSubst not extendIdSubst in another
  This was the root cause of Trac #11644

Minor refactoring

* Eliminate unused mkDerivedLocalCoVarM, mkUserLocalCoVar
* Small refactor in mkSysLocalOrCoVar