0.9.6

• #26 ULong.valueOf(String) accepts out-of-range decimal values
• #27 Reject out-of-range unsigned long strings
• #28 subtract wrongly throws when the result drops below 2⁶³
• #29 add(Long.MIN_VALUE) and subtract(Long.MIN_VALUE) cause a StackOverflowError
• #30 Consistently validate values on deserialisation
• #31 Consistently accept -0 as a valid value
• #32 ulong((BigInteger) null) should return null for consistency reasons