Add string operation for int256 in Strings.sol

[Suvadra-Barua]

🧐 Motivation
There is no implementation of the conversion of int256 to String

📝 Details
Using Strings.sol , We can convert uint256 to String,HexString and address to HexString. But int256 to string is required in many cases.

[ernestognw]

Partially solved in #3773, although there's no toHexString implementation

[Amxx]

What would the Hex representation of a negative integer be? If you want the 2-complement representation, just hard-cast the int256 to a uint256 and then do toHexString(uint256)

[ernestognw]

What would the Hex representation of a negative integer be?

I'd say there would be multiple Hex representations available, but that should be an standard so hold on for now.

[Amxx]

I'd say there should not be multiple representations available in our libraries. Any user is free to chose one they like, and implement it on top of what we already have.

[ernestognw]

I completely agree with not supporting them all, but I don't know about any way of generalizing it, and there's no standard usage afaik

just hard-cast the int256 to a uint256 and then do toHexString(uint256)

This is the most general version I'd think about. Can be converted to 2's complement.

[Amxx]

I'd like to says that I've never seen anyone write -0xE8A1 ... but I'd be lying.

[ernestognw]

Can you share a source (if any)? I haven't seen any two's complement on-chain, but maybe reaching out to people who's done it might be helpful

[Amxx]

I don't have any sources, because I have never seen anyone do int→string in solidity. Rather then go look for it, the potential users should come to us with realistic usecases.

When users ask for features, we ask them what their motivation/usecase is, and evaluate if that is relevant to the community in general.

[DigitalSubham]

I am working on this issue, please assign this to me

[Amxx]

@DigitalSubham

I'm not sure why this issue is not already closed, but we already have an implementation of toString(int256)