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.cpp

@@ -4,7 +4,7 @@
 #include "kmalloc.h"
 
 #ifndef __serenity__
-#include <new>
+#    include <new>
 #endif
 
 //#define DEBUG_STRINGIMPL
@@ -172,4 +172,19 @@ void StringImpl::compute_hash() const
     m_has_hash = true;
 }
 
+NonnullRefPtr<StringImpl> StringImpl::reversed() const
+{
+    if (m_length == 0)
+        return the_empty_stringimpl();
+
+    char* buffer;
+    const char* pos = &m_inline_buffer[m_length - 1];
+    auto new_impl = create_uninitialized(m_length, buffer);
+
+    for (int i = 0; i < m_length; i++)
+        buffer[i] = *pos--;
+
+    return new_impl;
+}
+
 }