Integer ergonomics changes

[chipshort]

• change Uint256::new to take a u128. It's much more common to need that for test code and even in production code, as opposed to providing the full byte array. Also, you can always use from_be_bytes instead. What's important here is that the function stays const
• do the analogous thing for Uint512, Int256 and Int512 (using i128 for the signed ints)