Compiler error instead of crash for too few args to constructor in case pattern.

[jemc]

This compiler crash occurs when the user accidentally used the non-let format of a case pattern, where the case is trying to instantiate the type and match with eq instead of matching on type,, and there was an earlier error when type-checking the case pattern expression (for example, too few arguments, as we have here..

class C
  new create(key: String) => None

actor Main
  new create(env: Env) =>
    let c: (C | None) = None
    match c
    | C => None
    end

This crash was originally encountered by @funkybob in #1878, but it was found to be a completely different bug.