Note [TyEqFlags]

When we call the eager unifier, e.g. through 'checkTyEqRhs', we specify what
kind of checks the unifier performs via the 'TyEqFlags' argument. In particular,
when the LHS type in a unification is a type variable, we might want to perform
different checks; this is achieved using the 'TEFTyVar' constructor to 'TyEqFlags':

  1. `notUnifying_TEFTask`
     LHS is a skolem tyvar, or an untouchable meta-tyvar.
     We are not unifying; we only want to perform occurs-checks.

      TEFTyVar
        { tefTyVar_occursCheck   = OC_Check ...
        , tefTyVar_levelCheck    = LC_None
        , tefTyVar_concreteCheck = CC_None
        , tef_fam_app            = TEFA_Recurse
        }

  2a. `unifyingLHSMetaTyVar_TEFTask`
     We are unifying; we want to perform an occurs check, a level check,
     and a concreteness check (when the meta-tyvar is a ConcreteTv).

      TEFTyVar
        { tefTyVar_occursCheck   = OC_Check ...
        , tefTyVar_levelCheck    = LC_Promote ...
        , tefTyVar_concreteCheck = CC_Promote or CC_None
            depending on whether or not the lhs tv is concrete
        , tef_fam_app            = TEFA_Recurse
        }

  2b. `defaulting_TEFTask`
     We are in the top-level defaulting code, considering unifying a
     touchable meta-tyvar with a type.

      TEFTyVar
        { tefTyVar_occursCheck   = OC_None
        , tefTyVar_levelCheck    = LC_None
        , tefTyVar_concreteCheck = CC_Promote or CC_None
            depending on whether or not the lhs tv is concrete
        , tef_fam_app            = TEFA_Recurse
        }

  3. `makeTypeConcrete`
     LHS is a fresh ConcreteTv meta-tyvar (see call to 'checkTyEqRhs' in
     `makeTypeConcrete`). We are unifying; we only want to perform
     a concreteness check.

      TEFTyVar
        { tefTyVar_occursCheck   = OC_None
        , tefTyVar_levelCheck    = LC_None
        , tefTyVar_concreteCheck = CC_Promote conc_orig
        , tef_fam_app            = TEFA_Recurse
        }

  4. `pureTyEqFlags_LHSMetaTyVar`
     We want to perform a non-monadic check, i.e. we want to know whether we are able
     to unify 'lhs_tv' with 'rhs_ty', but don't want to actually perform
     a side-effecting unification. This is used in 'mightEqualLater'.

      TEFTyVar
        { tefTyVar_occursCheck   = OC_Check ...
        , tefTyVar_levelCheck    = LC_Check ...
        , tefTyVar_concreteCheck = CC_Check or CC_None
            depending on whether or not the lhs tv is concrete
        , tef_fam_app            = TEFA_Recurse
        }

     Here 'LC_Check True' and 'ConcreteSimpleCheck' make 'checkTyEqRhs' perform
     conservative pure checks, without actually carrying out any promotion.