Add super_diff extension

[flash-gordon]

super_diff is very useful in combination with rspec. There's a simple way to integrate an existing class: adding attributes_for_super_diff method is enough.

Before:

       expected: #<Test::User name="Jane" age=22>
            got: #<Test::User name="Jane" age=21>

       (compared using eql?)

       Diff:
       @@ -1 +1 @@
       -#<Test::User name="Jane" age=22>
       +#<Test::User name="Jane" age=21>

After:

       expected: #<Test::User name: "Jane", age: 22>
            got: #<Test::User name: "Jane", age: 21>

       (compared using eql?)

       Diff:

       ┌ (Key) ──────────────────────────┐
       │ ‹-› in expected, not in actual  │
       │ ‹+› in actual, not in expected  │
       │ ‹ › in both expected and actual │
       └─────────────────────────────────┘

         #<Test::User {
           name: "Jane",
       -   age: 22
       +   age: 21
         }>

To use this extension, add super_diff:

Gemfile

group :test do
  gem 'super_diff'
end

Then activate the extension:
spec_helper.rb

require 'dry/struct'
Dry::Struct.load_extensions(:super_diff)