Scala 2.13 support

[gothug]

Would be terrific to have this library support Scala 2.13!

Scala 2.13 had a substantial revamp of collections library.

I tried to fork the code and make it support Scala 2.13, but found it quite not a trivial task. (Some interfaces, like SortedSetLike and GenSet has been dropped altogether in the new collections library).

Could not succeed this far in reorganising the code that heavily relies on the internal architecture of the previous generation of Scala collections.

[delenius]

I second this request. I was going to take a look at it too, before reading the comments above.