- 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
We never do 160-bit arithmetic.
Introduce new opaque implementation of `uint256`, move old "arithmetic" implementation to `arith_uint256.