Contextual enum variants

When an expected enum type is known, .Variant is enough for unit or payload construction. A function's return type gives its tail expression that context, just as a type annotation does for a binding. The same shorthand works in patterns, calls, imports, and aliases.

Without expected context, moon accepts the shorthand only when one visible enum owns that variant; otherwise it reports every candidate.

enum Signal { Go, Stop }

fn parse_label(signal: string) -> Signal {
  if signal == "go" { .Go } else { .Stop }
}

fn label(signal: Signal) -> string {
  match signal { .Go => "go", .Stop => "stop" }
}

fn main() {
  let signal: Signal = .Stop
  println(label(parse_label("go")))
  println(label(signal))
}