AK: Made Strings reversible

`AK::String` can now be reversed via AK::String::reverse(). This makes
life a lot easier for functions like `itoa()`, where the output
ends up being backwards. Very much not like the normal STL
(which requires an `std::reverse` object) way of doing things.

A call to reverse returns a new `AK::String` so as to not upset any
of the possible references to the same `StringImpl` shared between
Strings.

AK/StringImpl.h

@@ -1,7 +1,7 @@
 #pragma once
 
-#include <AK/RefPtr.h>
 #include <AK/RefCounted.h>
+#include <AK/RefPtr.h>
 #include <AK/Types.h>
 #include <AK/kmalloc.h>
 
@@ -44,6 +44,8 @@ public:
         return m_hash;
     }
 
+    NonnullRefPtr<StringImpl> reversed() const;
+
 private:
     enum ConstructTheEmptyStringImplTag {
         ConstructTheEmptyStringImpl