Adding SafeCast variants for signed integers

[julianmrodri]

#2240

Fixes #2240

Description: * Added SafeCast variants for signed Integers. This includes the following functions:

• toInt8(int256 x): Downcasts to int8 from int256
• toInt16(int256 x): Downcasts to int16 from int256
• toInt32(int256 x): Downcasts to int32 from int256
• toInt64(int256 x): Downcasts to int64 from int256
• toInt128(int256 x): Downcasts to int128 from int256

Also added the corresponding Tests and adjustments to the Mockup contract. All tests works correctly and linter is also validated.

Modified Files:

1. contracts/utils/SafeCast.sol
2. contracks/mocks/SafeCastMock.sol
3. test/utils/SafeCast.test.js

[julianmrodri]

@nventuro @frangio Ready to be reviewed. Let me know in case of any changes or modifications that need to be done. Thanks!

[nventuro]

This is awesome, thanks a lot @julianmrodri! Code style, tests, documentation, it's all perfect. I really don't have any comments other than an extremely minor suggestions. Do you have any questions or concerns about these new functions?

[julianmrodri]

Thanks @nventuro! I was sure I had to update the CHANGELOG as well but it looked quite sensitive as I wasn't sure I should include it under the next release, so I left it you to you guys.

I don't have any concerns about these functions I did quite a bunch of manual testing as well interacting with the Contract and seems to work as expected. The formula extrapolated nicely to all the different types.

Let me know if there's anything else to do or include and I can take care. Thanks again!

[nventuro]

Awesome, thanks! We don't ask contributors to edit the changelog to reduce the burden on them, specially since it's very simple for us to do from the web UI.

Thank you very much once more!