Scala 3 support for Seq / IndexedSeq

[OndrejSpanel]

PR for #90

Added tests which check Seq / IndexedSeq functionality (copied from List ones).

Tests currently pass on Scala 2 and fail on Scala 3.

I intend to implement the functionality within a few days.

[OndrejSpanel]

I have very simple solution which seems working, obtained by copying List functors for Seq / IndexedSeq. I hope to come with something more generic, but my Scala 3 experience is very limited (and frankly I am not very comfortable with higher-kinded types even in Scala 2, my head scratches when I think about them).

If someone has some advice / suggestion, or a link to a similar functionality in another library, feel free to chime in.

[OndrejSpanel]

I implemented a generic solution for any containers implementing Seq, copied from how Map is currently implemented. It looks generic enough and it passed my tests, therefore I think it is ready for review.

[adamw]

Thanks! Looks good :)