`Equatable` is redundant with `EquatableMixin`

[shilangyu]

Is your feature request related to a problem? Please describe.

When choosing which one to use (Equatable or EquatableMixin) there is no benefit of choosing Equatable (or I am failing to see one) over the mixin version. This duplicates the API for no clear reason to me.

Describe the solution you'd like

Removing the non-mixin version. This simplifies this package, docs, and usage.

Additional context

During code reviews I often have to tell people to use the mixin version instead, since it is more extensible (for example will allow in the future to extend some other class).

If I am missing some advantage that the class version has, please let me know what that is and feel free to close this issue.

[utamori]

I think this can be solved by making Equatable an Dart3 abstract mixin class. EquatableMixin should be deprecated and removed later. #161

[felangel]

Closing this since it will be resolved by #175