add Enum instance for Char, fix all versions

This is fully backward compatible, albeit with one caveat: it adds a dependency on the latest version of purescript-strings, so could potentially introduce a conflict if released under a patch fix update (albeit, only in the case a user of the library is not compatible with the latest purescript-strings and has opted not to fix patch version for their dependency on purescript-enums).

The current version is `0.2.1`, so this could be `0.2.2` under a patch fix update, or `0.3.0` otherwise.