DEV: Have contract take a block in services

Currently in services, the `contract` step is only used to define where
the contract will be called in the execution flow. Then, a `Contract`
class has to be defined with validations in it.

This patch allows the `contract` step to take a block containing
validations, attributes, etc. directly. No need to then open a
`Contract` class later in the service.

It also has a nice side effect, as it’s now easy to define multiples
contracts inside the same service. Before, we had the `class_name:`
option, but it wasn’t really useful as you had to redefine a complete
new contract class.
Now, when using a name for the contract other than `default`, a new
contract will be created automatically using the provided name.

Example:
```ruby
contract(:user) do
  attribute :user_id, :integer

  validates :user_id, presence: true
end
```
This will create a `UserContract` class and use it, also putting the
resulting contract in `context[:user_contract]`.

spec/lib/service/runner_spec.rb

@@ -38,18 +38,17 @@ RSpec.describe Service::Runner do
   class FailedContractService
     include Service::Base
 
-    class Contract
+    contract do
       attribute :test
+
       validates :test, presence: true
     end
-
-    contract
   end
 
   class SuccessContractService
     include Service::Base
 
-    contract
+    contract {}
   end
 
   class FailureWithModelService