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/Tests/TestString.cpp

@@ -41,6 +41,7 @@ TEST_CASE(compare)
     EXPECT(!("a" >= String("b")));
     EXPECT("a" <= String("a"));
     EXPECT(!("b" <= String("a")));
+    EXPECT(!strcmp(test_string.reversed().characters(), "FEDCBA"));
 }
 
 TEST_CASE(index_access)