More flexible record typing

[soutaro]

• Support alias to record types (bug fix!)
• Record types are now co-variant
• Allows skipping nil-able attributes

# @type var x: { name: String, email: String? }
x = { name: "foo", email: "foo@example.com" }          # OK
x = { name: "foo", email: nil }                        # OK
x = { name: "foo" }                                    # OK 🐕