DEV: Drop OpenStruct for the context object in services

While using `OpenStruct` is nice, it’s generally not a very good idea as
it usually leads to performance problems.

The `OpenStruct` source code even says basically to avoid it.

Since the context object is crucial in our services, this patch replaces
`OpenStruct` with a custom implementation instead.

spec/lib/service/steps_inspector_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-RSpec.describe StepsInspector do
+RSpec.describe Service::StepsInspector do
   class DummyService
     include Service::Base
 
@@ -239,7 +239,7 @@ RSpec.describe StepsInspector do
       end
 
       context "when a reason is provided" do
-        before { result["result.policy.policy"].reason = "failed" }
+        before { result["result.policy.policy"][:reason] = "failed" }
 
         it "returns the reason" do
           expect(error).to eq "failed"

spec/support/service_matchers.rb

@@ -26,7 +26,7 @@ module ServiceMatchers
     private
 
     def error_message_with_inspection(message)
-      inspector = StepsInspector.new(result)
+      inspector = Service::StepsInspector.new(result)
       "#{message}\n\n#{inspector.inspect}\n\n#{inspector.error}"
     end
   end
@@ -89,7 +89,7 @@ module ServiceMatchers
     end
 
     def error_message_with_inspection(message)
-      inspector = StepsInspector.new(result)
+      inspector = Service::StepsInspector.new(result)
       "#{message}\n\n#{inspector.inspect}\n\n#{inspector.error}"
     end
 
@@ -158,7 +158,7 @@ module ServiceMatchers
   end
 
   def inspect_steps(result)
-    inspector = StepsInspector.new(result)
+    inspector = Service::StepsInspector.new(result)
     puts "Steps:"
     puts inspector.inspect
     puts "\nFirst error:"