Print Lists consistently in exhaustivity warnings

[abgruszecki]

First off - I wasn't too sure if I should open an issue for this, since it's so trivial. I apologise if I should.

This PR adjusts how Lists are printed in error messages. Previously, depending on the specific pattern match in question, an unmatched :: would print differently depending on whether we ended up with a Typ space or Kon space. For the simplest example, Typ(::) was printed as List(_), but Kon(::, Int, List[Int]) was printed as List(_, _*). The error messages were also slightly confusing, since to me List(_) suggests that the list has only one member, whereas a :: can have one-or-more members. After adjustments, List now prints consistently as though we had ended up with the Kon space. Note that the code is equivalent to showing Kon(tp, signature(tp).map(Typ(_, false))).

/cc @liufengyun

[smarter]

First off - I wasn't too sure if I should open an issue for this, since it's so trivial. I apologise if I should.

It's fine to open PRs without corresponding issues :).

[liufengyun]

Thanks @AleksanderBG , LGTM!