String == String does not work with embedded NUL characters

https://github.com/arduino/ArduinoCore-API/blob/7f8de5869ee3de7a915bf1db64313c08595b3ac3/api/String.cpp#L445 and https://github.com/arduino/ArduinoCore-API/blob/7f8de5869ee3de7a915bf1db64313c08595b3ac3/api/String.cpp#L445 should use ``memcmp()`` to make sure that a String object can be compared to another String object that has embedded ``NUL`` characters

Start in api/String.cpp at the comparison code around line 445 and inspect how String objects are compared when embedded NUL characters are present. Update the comparison as described to handle those characters, then verify that String-to-String comparisons work correctly for embedded NUL values.

Written by the indexing model from the issue text.