add `Char` module

[aspeddro]

Perhaps add chrOpt, return option<chr>, @zth, @fhammerschmidt?

TODO:

• [ ] Get rid of PervasivesU
• [ ] Add docstrings
• [ ] Update CHANGELOG.md

Close #228

[fhammerschmidt]

Yes, a chrOpt that checks if a given int is in the valid char range and only then returns a Some(char) is missing currently, imo.