Revert "Can simplify InternedStringPool"

This reverts commit c78fdcb9.

src/core/stringpool.cpp

@@ -19,8 +19,19 @@
 namespace pyston {
 
 InternedString InternedStringPool::get(llvm::StringRef arg) {
-    // HACK: should properly track this liveness:
-    BoxedString* s = internStringImmortal(arg);
+    auto it = interned.find(arg);
+
+    BoxedString* s;
+    if (it != interned.end()) {
+        s = it->second;
+    } else {
+        // HACK: should properly track this liveness:
+        s = internStringImmortal(arg);
+
+        // Note: make sure the key points to the value we just created, not the
+        // argument string:
+        interned[s->s()] = s;
+    }
 
 #ifndef NDEBUG
     return InternedString(s, this);

src/core/stringpool.h

@@ -89,6 +89,12 @@ public:
 };
 
 class InternedStringPool {
+private:
+    // We probably don't need to pull in llvm::StringRef as the key, but it's better than std::string
+    // which I assume forces extra allocations.
+    // (We could define a custom string-pointer container but is it worth it?)
+    std::unordered_map<llvm::StringRef, BoxedString*> interned;
+
 public:
     void gcHandler(gc::GCVisitor* v);
     InternedString get(llvm::StringRef s);