Restrict some boolean properties to Bool in the compiler

[HertzDevil]

Instead of using !! inside, the caller has to do this explicitly. (The compiler itself doesn't pass arguments that aren't already Bool.)

Note that the following is valid code, but infers Foo#@foo to be a Bool | Nil rather than Bool:

class Foo
  # no other declarations of `@foo` here

  def foo=(foo)
    @foo = !!foo
  end
end

This happened to Crystal::CodeError#@warning.