Fixups for joroKr21's abstract-types PR

[carymrobbins]

Brings together both of our changes for resolving refs estatico#8 which also includes my introduction of cats-tests

[carymrobbins]

@joroKr21 If you merge these changes into your branch, I'll go ahead and merge your branch upstream and cut a 0.3.0 release. Thanks for your help!

[carymrobbins]

Also note that I've removed the <: usage in BaseNewType. That's a legacy encoding and I think it's less important for that to be there. It seems to not work well when using NewTypeOps, specifically complains about type invariance with NewTypeAutoOps#toNewTypeOps -

[error] scala-newtype/shared/src/main/scala/io/estatico/newtype/NewType.scala:17: type mismatch;
[error]  found   : x.type (with underlying type NewTypeAutoOps.this.Type)
[error]  required: io.estatico.newtype.BaseNewType.Aux[NewTypeAutoOps.this.Type,NewTypeAutoOps.this.Tag,NewTypeAutoOps.this.Repr]
[error]     (which expands to)  io.estatico.newtype.BaseNewType.Aux[io.estatico.newtype.BaseNewType.Aux[NewTypeAutoOps.this.Base,NewTypeAutoOps.this.Tag,NewTypeAutoOps.this.Repr],NewTypeAutoOps.this.Tag,NewTypeAutoOps.this.Repr]
[error] Note: NewTypeAutoOps.this.Base >: NewTypeAutoOps.this.Type, but type Aux is invariant in type B.
[error] You may wish to define B as -B instead. (SLS 4.5)
[error]   ): NewTypeOps[Type, Tag, Repr] = new NewTypeOps[Type, Tag, Repr](x)
[error]                                                                    ^

I assume that since we've introduced the @newtype macro, the need to rely on the legacy encoding will be less and less as time marches on.

[carymrobbins]

Actually, I think I can just change the base of your PR to my branch, that way we can retain the history a bit better (especially since I'm introducing cats-tests).

[carymrobbins]

Also note that I've removed the <: usage in BaseNewType. That's a legacy encoding and I think it's less important for that to be there. It seems to not work well when using NewTypeOps, specifically complains about type invariance with NewTypeAutoOps#toNewTypeOps -

Actually, that may have been before I introduced your additional fix which corrected the type params applied in NewTypeAutoOps. You can see the final solution in my estatico#9 PR, which just applies the same "have object inherit from a trait" trick.

https://github.com/estatico/scala-newtype/pull/9/files#diff-0cc28c2b4e8123ead5d9bf361f2ae32bL23