AK: Add FlyString, a simple flyweight string class

FlyString is a flyweight string class that wraps a RefPtr<StringImpl>
known to be unique among the set of FlyStrings. The class is very
unoptimized at the moment.

When to use FlyString:

- When you want O(1) string comparison
- When you want to deduplicate a lot of identical strings

When not to use FlyString:

- For strings that don't need either of the above features
- For strings that are likely to be unique

AK/Tests/Makefile

@@ -7,6 +7,7 @@ SHARED_TEST_SOURCES = \
 	../LogStream.cpp \
 	../JsonValue.cpp \
 	../JsonParser.cpp \
+    ../FlyString.cpp \
     ../FileSystemPath.cpp \
     ../URL.cpp \
     ../Utf8View.cpp

AK/Tests/TestString.cpp

@@ -26,7 +26,9 @@
 
 #include <AK/TestSuite.h>
 
+#include <AK/FlyString.h>
 #include <AK/String.h>
+#include <AK/StringBuilder.h>
 
 TEST_CASE(construct_empty)
 {
@@ -137,4 +139,24 @@ TEST_CASE(to_uppercase)
     EXPECT(String("AbC").to_uppercase() == "ABC");
 }
 
+TEST_CASE(flystring)
+{
+    {
+        FlyString a("foo");
+        FlyString b("foo");
+        EXPECT_EQ(a.impl(), b.impl());
+    }
+
+    {
+        String a = "foo";
+        FlyString b = a;
+        StringBuilder builder;
+        builder.append('f');
+        builder.append("oo");
+        FlyString c = builder.to_string();
+        EXPECT_EQ(a.impl(), b.impl());
+        EXPECT_EQ(a.impl(), c.impl());
+    }
+}
+
 TEST_MAIN(String)