code: public static Pair<String, String> stringIntern(Pair<String, String> p) {
      return new MutableInternedPair(p);
    }
docs: If first and second are Strings, then this returns an MutableInternedPair
  where the Strings have been interned, and if this Pair is serialized
  and then deserialized, first and second are interned upon
  deserialization.
  
  @param p A pair of Strings
  @return MutableInternedPair, with same first and second as this.