Remove initialization from vector (as this is only used in the tests).
Also implement SetHex and GetHex in terms of uint256, to avoid
duplicate code as well as avoid endianness issues (as they
work in term of bytes).
- Methods that access the guts of arith_uint256 are removed,
as these are incompatible between endians. Use uint256 instead
- Serialization is no longer needed as arith_uint256's are never
read or written
- GetHash is never used on arith_uint256
