Support I256 in format_units and parse_units

[mds1]

format_units and parse_units only support U256 and are do not let you use I256 as the input or output type

I think these methods should probably take/return something similar to enum BigInt { Int(I256), Uint(U256) } and match on that. I'm not sure if additional enum types are required to support smaller input types like u128, i128, etc.

Support for this would make full signed integer support in cast much simpler. This is also required to fix the LogNamedDecimalIntFilter method in forge