Converting integers to string as if unsigned

Josh writes:

Today a colleague of mine needed to translate a long to a string as if
it were unsigned. java.lang.Long lets you do this in binary, octal,
or hex, but not decimal. I had to give him this piece of inefficient
code that leverages BigIntegers string conversion capabilities:

  public static String ulongToString(long i) {
    BigInteger b = BigInteger.valueOf(i & Long.MAX_VALUE);
    if (i < 0)
      b = b.flipBit(Long.SIZE - 1);
    return b.toString();
  }

Surely we don't want people doing this! As we've discussed over the
years, Integer, Long, and Short should have a bunch of methods for
dealing with integeral primitives as if they were unsigned. We don't
need many. If we could only have one, it would be the one above
(coded efficiently, of course). But I think we should also have some
upconversion operations (ushort -> int, uint -> long), arithmetic
operations (most but not all of which are simple pass-throughs to the
signed operations), and comparison operations (which rely on the ^
MAX_VALUE trick). Can we please have these in Dolphin? I am willing
to write them.