LibJS: Keep track of PrimitiveStrings and share them

VM now has a string cache which tracks all live PrimitiveStrings and
reuses an existing one if possible. This drastically reduces the number
of GC-allocated strings in many real-word situations.

Userland/Libraries/LibJS/Heap/Heap.cpp

@@ -52,6 +52,7 @@ Heap::Heap(VM& vm)
 
 Heap::~Heap()
 {
+    vm().string_cache().clear();
     collect_garbage(CollectionType::CollectEverything);
 }
 

Userland/Libraries/LibJS/Runtime/PrimitiveString.cpp

@@ -25,6 +25,7 @@ PrimitiveString::PrimitiveString(Utf16String string)
 
 PrimitiveString::~PrimitiveString()
 {
+    vm().string_cache().remove(m_utf8_string);
 }
 
 String const& PrimitiveString::string() const
@@ -90,7 +91,14 @@ PrimitiveString* js_string(Heap& heap, String string)
             return &heap.vm().single_ascii_character_string(ch);
     }
 
-    return heap.allocate_without_global_object<PrimitiveString>(move(string));
+    auto& string_cache = heap.vm().string_cache();
+    auto it = string_cache.find(string);
+    if (it == string_cache.end()) {
+        auto* new_string = heap.allocate_without_global_object<PrimitiveString>(string);
+        string_cache.set(move(string), new_string);
+        return new_string;
+    }
+    return it->value;
 }
 
 PrimitiveString* js_string(VM& vm, String string)

Userland/Libraries/LibJS/Runtime/VM.h

@@ -81,6 +81,7 @@ public:
 
     Symbol* get_global_symbol(const String& description);
 
+    HashMap<String, PrimitiveString*>& string_cache() { return m_string_cache; }
     PrimitiveString& empty_string() { return *m_empty_string; }
     PrimitiveString& single_ascii_character_string(u8 character)
     {
@@ -294,6 +295,8 @@ private:
 
     Exception* m_exception { nullptr };
 
+    HashMap<String, PrimitiveString*> m_string_cache;
+
     Heap m_heap;
     Vector<Interpreter*> m_interpreters;