Handle Bool with exhaustive match

[SeanTAllen]

Currently, the following code isn't seen as an exhaustive match:

actor Main
	new create(env: Env) =>
	  env.out.print("Hello World")

	fun box fourty_two(err: Bool = false): USize ? =>
		match err
	  | true =>
	    return 50
	  | false =>
	    return 42
	  end

This is surprising to folks and we believe that exhaustiveness checking should be enhanced to handle this for booleans.