(experimental) Split Array and MutableArray

[yhara]

Motivation

• To encourage using immutable data structures
• Immutable Array may be more convenient in some cases; For example

class Shape; ...; end
class Circle : Shape; ...; end
class Rect : Shape; ...; end
class Renderer
  def self.render(shapes: Array<Shape>); ...; end
end

# This is inferred as `Array<Circle>`
let circles = [Circle.new(1), Circle.new(2)]
# This is type error now - but this will be safe if `Array` is immutable.
Renderer.render(circles)