Struct both from JSON and initialize

---

examples/struct/struct_initialize_and_from_json.cr

require "json"

struct Person
  include JSON::Serializable

  getter name : String
  getter email : String

  def initialize(@name, @email)
  end
end

prs1 = Person.new("Foo", "me@foo.bar")
p! prs1
p! prs1.name
p! prs1.email

json_str = %{{"name": "Bar", "email": "bar@foobar.com"}}
prs2 = Person.from_json(json_str)
p! prs2
p! prs2.name
p! prs2.email

prs1 # => Person(@name="Foo", @email="me@foo.bar")
prs1.name # => "Foo"
prs1.email # => "me@foo.bar"
prs2 # => Person(@name="Bar", @email="bar@foobar.com")
prs2.name # => "Bar"
prs2.email # => "bar@foobar.com"