Restore old (1.6.5) NaturalWeight

Because of a conflict with Kaldi's specialization on CompactLatticeWeightTpl,
https://groups.google.com/forum/#!topic/kaldi-developers/RuH8Z1Ttuuk

There is a pending PR to remove the dependency:
kaldi-asr/kaldi#2313

Also rename one type alias in arc-map.h because of a name hiding warning.

src/include/fst/arc-map.h

@@ -914,10 +914,10 @@ class FromGallicMapper {
   template <GallicType GT>
   static bool Extract(const GallicWeight<Label, AW, GT> &gallic_weight,
                       typename A::Weight *weight, typename A::Label *label) {
-    using GW = StringWeight<Label, GallicStringType(GT)>;
-    const GW &w1 = gallic_weight.Value1();
+    using GWT = StringWeight<Label, GallicStringType(GT)>;
+    const GWT &w1 = gallic_weight.Value1();
     const AW &w2 = gallic_weight.Value2();
-    typename GW::Iterator iter1(w1);
+    typename GWT::Iterator iter1(w1);
     const Label l = w1.Size() == 1 ? iter1.Value() : 0;
     if (l == kStringInfinity || l == kStringBad || w1.Size() > 1) return false;
     *label = l;

src/include/fst/weight.h

@@ -169,37 +169,20 @@ enum DivideType {
 //
 // We define the strict version of this order below.
 
-// Declares the template with a second parameter determining whether or not it
-// can actually be constructed.
-template <class W, class IdempotentType = void>
-class NaturalLess;
-
-// Variant for idempotent weights.
-template <class W>
-class NaturalLess<W, typename std::enable_if<IsIdempotent<W>::value>::type> {
- public:
-  using Weight = W;
-
-  NaturalLess() {}
-
-  bool operator()(const Weight &w1, const Weight &w2) const {
-    return w1 != w2 && Plus(w1, w2) == w1;
-  }
-};
-
-// Non-constructible variant for non-idempotent weights.
 template <class W>
-class NaturalLess<W, typename std::enable_if<!IsIdempotent<W>::value>::type> {
- public:
+class NaturalLess {
+public:
   using Weight = W;
 
-  // TODO(kbg): Trace down anywhere this is being instantiated, then add a
-  // static_assert to prevent this from being instantiated.
   NaturalLess() {
-    FSTERROR() << "NaturalLess: Weight type is not idempotent: " << W::Type();
+    if (!(W::Properties() & kIdempotent)) {
+      FSTERROR() << "NaturalLess: Weight type is not idempotent: " << W::Type();
+    }
   }
 
-  bool operator()(const Weight &, const Weight &) const { return false; }
+  bool operator()(const W &w1, const W &w2) const {
+    return (Plus(w1, w2) == w1) && w1 != w2;
+  }
 };
 
 // Power is the iterated product for arbitrary semirings such that Power(w, 0)