Javalib `Set.scala` is missing a number of methods.

[LeeTibbert]

This may read like a whine, but it is really a "forget-me-not".

Whilst working on what is now PR #3997, I became aware, like a freight car hitting you, that javalib Set.scala is missing a number of methods. The methods are there, but commented out by a Todo.

IIRc, equals and hashcode are among the commented-out methods. That can never be a good sign.

It is true that, to my knowledge, no one has complained here about those missing declarations in the eight or so years this has been true. It would be nice to get ahead of the curve and not wait for complaints.