Re-implement selector unification

This new implementation is similar to the one in dart-sass.

Also fixes a few bugs in hash calculation (`==` objects having different hashes) and comparison (`a < b` and `b < a` being true at the same time; this should also fix #2776).

The following tests are fixed by this change:

```
/spec/selector-functions/unify/universal_simple
/spec/extend-tests/237_extend_with_universal_selector_different_namespace
/spec/extend-tests/040_test_universal_unification_with_namespaced_element_target
/spec/extend-tests/053_test_element_unification_with_namespaced_universal_target
/spec/extend-tests/236_extend_with_universal_selector_empty_namespace
/spec/extend-tests/096_test_long_extender_runs_unification
/spec/extend-tests/060_test_element_unification_with_namespaceless_element_target
/spec/extend-tests/051_test_element_unification_with_namespaceless_universal_target
/spec/extend-tests/038_test_universal_unification_with_namespaceless_element_target
/spec/extend-tests/029_test_universal_unification_with_namespaceless_universal_target
/spec/extend-tests/031_test_universal_unification_with_namespaced_universal_target
/spec/extend-tests/062_test_element_unification_with_namespaced_element_target
```

sass-spec output_styles update: https://github.com/sass/sass-spec/pull/1319/files

include/sass/base.h

@@ -1,6 +1,7 @@
 #ifndef SASS_BASE_H
 #define SASS_BASE_H
 
+// #define DEBUG
 // #define DEBUG_SHARED_PTR
 
 #ifdef _MSC_VER

src/ast.hpp

@@ -319,6 +319,12 @@ namespace Sass {
       return hash_;
     }
 
+    template <typename P, typename V>
+    typename std::vector<T>::iterator insert(P position, const V& val) {
+      reset_hash();
+      return elements_.insert(position, val);
+    }
+
     typename std::vector<T>::iterator end() { return elements_.end(); }
     typename std::vector<T>::iterator begin() { return elements_.begin(); }
     typename std::vector<T>::const_iterator end() const { return elements_.end(); }

src/ast_fwd_decl.hpp

@@ -431,7 +431,6 @@ namespace Sass {
   typedef std::pair<Complex_Selector_Obj, SubSetMapPairs> SubSetMapResult;
   typedef std::vector<SubSetMapResult> SubSetMapResults;
 
-  #define OrderSelectors OrderFunction<Selector_Obj>
   typedef std::set<Selector_Obj, OrderNodes> SelectorSet;
 
   typedef std::deque<Complex_Selector_Obj> ComplexSelectorDeque;

src/ast_sel_cmp.cpp

@@ -467,10 +467,7 @@ namespace Sass {
 
   bool Simple_Selector::operator== (const Selector_List& rhs) const
   {
-    size_t len = rhs.length();
-    if (len > 1) return false;
-    if (len == 0) return empty();
-    return *this == *rhs.at(0);
+    return rhs.length() == 1 && *this == *rhs.at(0);
   }
 
   bool Simple_Selector::operator< (const Selector_List& rhs) const
@@ -483,9 +480,9 @@ namespace Sass {
 
   bool Simple_Selector::operator== (const Complex_Selector& rhs) const
   {
-    if (rhs.tail()) return false;
-    if (!rhs.head()) return empty();
-    return *this == *rhs.head();
+    return !rhs.tail() && rhs.head() &&
+           rhs.combinator() == Complex_Selector::ANCESTOR_OF &&
+           *this == *rhs.head();
   }
 
   bool Simple_Selector::operator< (const Complex_Selector& rhs) const
@@ -497,10 +494,7 @@ namespace Sass {
 
   bool Simple_Selector::operator== (const Compound_Selector& rhs) const
   {
-    size_t len = rhs.length();
-    if (len > 1) return false;
-    if (len == 0) return empty();
-    return *this == *rhs.at(0);
+    return rhs.length() == 1 && *this == *rhs.at(0);
   }
 
   bool Simple_Selector::operator< (const Compound_Selector& rhs) const
@@ -832,15 +826,11 @@ namespace Sass {
 
   bool Type_Selector::operator< (const Type_Selector& rhs) const
   {
-    if (is_ns_eq(rhs))
-    { 
-      if (rhs.has_ns_ && has_ns_)
-        return name() < rhs.name();
-      if (!rhs.has_ns_ && !has_ns_)
-        return name() < rhs.name();
-      return true;
-    }
-    return ns() < rhs.ns();
+    return has_ns_ == rhs.has_ns_
+      ? (ns_ == rhs.ns_
+         ? name_ < rhs.name_
+         : ns_ < rhs.ns_)
+      : has_ns_ < rhs.has_ns_;
   }
 
   bool Class_Selector::operator< (const Class_Selector& rhs) const
@@ -912,4 +902,4 @@ namespace Sass {
   /*#########################################################################*/
   /*#########################################################################*/
 
-}
+}