Optimize unique_* methods

[jgaskins]

Set is faster for inclusion checks than Array — O(1) vs O(n). When generating a lot of fake data that you need to be unique, it makes a pretty big difference. I discovered this while creating 100k test users in my DB with a unique constraint on email addresses.

It even makes a huge difference for this test suite. The difference in runtime on my machine for the generate unique result test is 656ms vs 49ms, or 12x as fast.

Before this commit:

.........................................................................................

Top 10 slowest examples (680m seconds, 99.01% of total time):
  Faker::Address should generate unique result
    656m seconds spec/address_spec.cr:25
  Faker::Internet user_name_with_closed_range_arg
    6.39m seconds spec/internet_spec.cr:30
  Faker::Internet user_name_with_open_range_arg
    6.19m seconds spec/internet_spec.cr:40
  Faker::Internet user_name_with_range_and_separators
    5.94m seconds spec/internet_spec.cr:50
  Faker::Lorem should return deterministic results when seeded
    1.43m seconds spec/lorem_spec.cr:4
  Faker::Internet ip_v4_address
    1.21m seconds spec/internet_spec.cr:105
  Faker::Internet should return deterministic results when seeded
    819µ seconds spec/internet_spec.cr:170
  Faker regexify
    643µ seconds spec/faker_spec.cr:14
  Faker::Internet password_max_with_integer_arg
    512µ seconds spec/internet_spec.cr:70
  Faker::Internet mac_address
    479µ seconds spec/internet_spec.cr:130
  Faker::Internet ip_v6_address
    462µ seconds spec/internet_spec.cr:142

Finished in 686.49 milliseconds
89 examples, 0 failures, 0 errors, 0 pending

After this commit:

.........................................................................................

Top 10 slowest examples (73.1m seconds, 90.82% of total time):
  Faker::Address should generate unique result
    48.5m seconds spec/address_spec.cr:25
  Faker::Internet user_name_with_open_range_arg
    6.37m seconds spec/internet_spec.cr:40
  Faker::Internet user_name_with_closed_range_arg
    6.21m seconds spec/internet_spec.cr:30
  Faker::Internet user_name_with_range_and_separators
    5.83m seconds spec/internet_spec.cr:50
  Faker::Lorem should return deterministic results when seeded
    2.06m seconds spec/lorem_spec.cr:4
  Faker::Internet ip_v4_address
    1.25m seconds spec/internet_spec.cr:105
  Faker::Internet should return deterministic results when seeded
    742µ seconds spec/internet_spec.cr:170
  Faker regexify
    608µ seconds spec/faker_spec.cr:14
  Faker::Internet ip_v6_address
    514µ seconds spec/internet_spec.cr:142
  Faker::Internet password_max_with_integer_arg
    514µ seconds spec/internet_spec.cr:70
  Faker::Internet mac_address
    478µ seconds spec/internet_spec.cr:130

Finished in 80.49 milliseconds
89 examples, 0 failures, 0 errors, 0 pending

[robacarp]

Seems like a clean change, and well proven too. Thanks @jgaskins

[askn]

thanks @jgaskins 🎉