Make num256 types implement copy

[jkilpatr]

Even if we have to do the bit fiddling ourselves.

[mpapierski]

So the general consensus on having Copy trait in Int256/Uint256 is that we're using num-bigint which uses dynamic memory which in current rust makes the type noncopyable.

Possible alternative solutions:

• Implement our own type using 8x u32 - Something like Bitcoin Core's https://github.com/bitcoin/bitcoin/blob/master/src/arith_uint256.cpp should work fine.
• Take ethereum-types and make it work on big endian.

[jkilpatr]

resolved #19