AK: Add String::number() for creating a String from a number.

Instead of manually doing String::format("%d"/"%u") everywhere, let's have
a String API for this. It's just a wrapper around format() for now, but it
could be made more efficient in the future.

LibCore/CProcessStatisticsReader.cpp

@@ -54,5 +54,5 @@ String CProcessStatisticsReader::get_username_from_uid(const uid_t uid)
     if (it != m_usernames.end())
         return (*it).value;
     else
-        return String::format("%u", uid);
+        return String::number(uid);
 }