Parse 0x8 and -0x8 correctly

[lynn]

A nasty hidden behavior of BigInteger.TryParse was making 0x8 parse as -8:

If value is a hexadecimal string, the TryParse(String, NumberStyles, IFormatProvider, BigInteger) method interprets value as a negative number stored by using two's complement representation if its first two hexadecimal digits are greater than or equal to 0x80. In other words, the method interprets the highest-order bit of the first byte in value as the sign bit. To make sure that a hexadecimal string is correctly interpreted as a positive number, the first digit in value must have a value of zero.

This PR makes 0x8 parse as 8, and -0x8 (previously unsupported) parse as -8.

[emily33901]

Looks good to me