Implement extend memoize on the compound selector level

We obviously need to return clones from the function.

src/extend.cpp

@@ -1511,6 +1511,13 @@ namespace Sass {
   };
   Node Extend::extendCompoundSelector(Compound_Selector_Ptr pSelector, CompoundSelectorSet& seen, bool isReplace) {
 
+    // check if we already extended this selector
+    // we can do this since subset_map is "static"
+    auto memoized = memoizeCompound.find(pSelector);
+    if (memoized != memoizeCompound.end()) {
+      return memoized->second.klone();
+    }
+
     DEBUG_EXEC(EXTEND_COMPOUND, printCompoundSelector(pSelector, "EXTEND COMPOUND: "))
     // TODO: Ruby has another loop here to skip certain members?
 
@@ -1658,6 +1665,9 @@ namespace Sass {
 
     DEBUG_EXEC(EXTEND_COMPOUND, printCompoundSelector(pSelector, "EXTEND COMPOUND END: "))
 
+    // memory results in a map table - since extending is very expensive
+    memoizeCompound.insert(std::pair<Compound_Selector_Obj, Node>(pSelector, results));
+
     return results;
   }
 

src/extend.hpp

@@ -36,6 +36,13 @@ namespace Sass {
       CompareNodes // compare
     > memoizeComplex;
 
+    std::unordered_map<
+      Compound_Selector_Obj, // key
+      Node, // value
+      HashNodes, // hasher
+      CompareNodes // compare
+    > memoizeCompound;
+
     void extendObjectWithSelectorAndBlock(Ruleset_Ptr pObject);
     Node extendComplexSelector(Complex_Selector_Ptr sel, CompoundSelectorSet& seen, bool isReplace, bool isOriginal);
     Node extendCompoundSelector(Compound_Selector_Ptr sel, CompoundSelectorSet& seen, bool isReplace);