* Add signed to unsigned conversion to SafeCast

* Update SafeCast exception message

* Add test for SafeCast int to uint conversion

- Update SafeCastMock
- Add tests for SafeCast int256 to uint256

* Update SafeCast int to uint definition

Apply suggestions from code review.

Co-Authored-By: Nicolás Venturo <nicolas.venturo@gmail.com>

* Update test for SafeCast int to uint conversion

* Update SafeCast test after code review

- Change "downcasts" to "casts"
- Move test closer to its function

* Fix error in SafeCast toUint256 description

* Fix breaking error in SafeCast

* Add uint256 to int256 conversion to SafeCast

- Add function
- Add mock
- Add test

* Update SafeCast unsigned to signed conversion

- Update error in conversion to be more clear
- Update constants in test to be powers of 2 instead of shifts

* Add changelog entry

* Update SafeCast tests

- Add minus in INT256_MIN for clarity

Co-Authored-By: Nicolás Venturo <nicolas.venturo@gmail.com>

Co-authored-by: Nicolás Venturo <nicolas.venturo@gmail.com>