Using `singleton` on variables

It would be useful to be able to apply singleton to type variables as well as concrete types. As a motivating example consider this simple event bus:

interface _Subscriber[E]
  def call: (E event) -> void
end

class EventBus
  def subscribe: [E] (_Subscriber[E] subscriber, to: singleton(E)) -> void
  def dispatch: (untyped event) -> void
end

Here I want to specify that EventBus#subscribe expects the subscriber to handle events of the type given as the to: argument.

No file or test path is named. Start by locating RBS's existing handling for singleton types and type variables, using the EventBus signature as the reproducer. Done means singleton can express the type variable from the to: argument and the subscriber/event relationship is covered by tests.