AK: Add template specializations for Optional<{,Fly}String>

Slice the size of `Optional<{,Fly}String>` in half by introducing
`UINTPTR_MAX` as an invalid bit pattern for these values.

AK/FlyString.cpp

@@ -50,6 +50,8 @@ FlyString FlyString::from_utf8_without_validation(ReadonlyBytes string)
 
 FlyString::FlyString(String const& string)
 {
+    ASSERT(!string.is_invalid());
+
     if (string.is_short_string()) {
         m_data = string;
         return;