Add missing `require "set"`
Add missing `require "set"`

```console
$ ruby -r strong_json -e 'StrongJSON.new { let :a, object }'
Traceback (most recent call last):
	5: from -e:1:in `<main>'
	4: from -e:1:in `new'
	3: from /usr/local/lib/ruby/gems/2.7.0/gems/strong_json-2.1.2/lib/strong_json.rb:9:in `initialize'
	2: from /usr/local/lib/ruby/gems/2.7.0/gems/strong_json-2.1.2/lib/strong_json.rb:9:in `instance_eval'
	1: from -e:1:in `block in <main>'
/usr/local/lib/ruby/gems/2.7.0/gems/strong_json-2.1.2/lib/strong_json/types.rb:5:in `object': uninitialized constant StrongJSON::Types::Set (NameError)
```

soutaro/strong_json