Multi-level struct from JSON

---

examples/struct/multi_level_struct_from_json.cr

require "json"

struct Address
  include JSON::Serializable

  getter street : String
  getter city : String
  getter country : String
end

struct Person
  include JSON::Serializable

  getter name : String
  getter email : String
  getter address : Address
end

json_str = %[{
  "name": "Bar",
  "email": "bar@foobar.com",
  "address" : {
    "street": "Broadway",
    "city": "New York",
    "country": "USA"
   }
}]
prs = Person.from_json(json_str)
pp! prs

p! prs.name
p! prs.address
p! prs.address.street

prs # => Person(
 @address=Address(@city="New York", @country="USA", @street="Broadway"),
 @email="bar@foobar.com",
 @name="Bar")
prs.name # => "Bar"
prs.address # => Address(@street="Broadway", @city="New York", @country="USA")
prs.address.street # => "Broadway"