Add #call method to each result

This feels like a good middle-ground.
I didn't want this to be a factory method, or return an instance.
That seemed like overkill for formatting.
But I don't want to place if and case-statements everywhere in the 
formatters.
I hope this doesn't violate single-responsibility principle or any other 
guidelines.

src/spectator/errored_result.cr

@@ -6,5 +6,9 @@ module Spectator
   # This is different from a "failed" result
   # in that the error was not from a failed expectation.
   class ErroredResult < FailedResult
+    # Calls the `error` method on `interface` and passes `self`.
+    def call(interface)
+      interface.error(self)
+    end
   end
 end

src/spectator/failed_result.cr

@@ -18,5 +18,10 @@ module Spectator
     def initialize(example, elapsed, @expectations, @error)
       super(example, elapsed)
     end
+
+    # Calls the `failure` method on `interface` and passes `self`.
+    def call(interface)
+      interface.failure(self)
+    end
   end
 end

src/spectator/pending_result.cr

@@ -5,5 +5,9 @@ module Spectator
   # A pending result means the example is not ready to run yet.
   # This can happen when the functionality to be tested is not implemented yet.
   class PendingResult < Result
+    # Calls the `pending` method on `interface` and passes `self`.
+    def call(interface)
+      interface.pending(self)
+    end
   end
 end

src/spectator/result.cr

@@ -10,5 +10,10 @@ module Spectator
     # and that this result is for.
     def initialize(@example)
     end
+
+    # Calls the corresponding method for the type of result.
+    # This is used to avoid placing if or case-statements everywhere based on type.
+    # Each sub-class implements this method by calling the correct method on `interface`.
+    abstract def call(interface)
   end
 end

src/spectator/successful_result.cr

@@ -14,5 +14,10 @@ module Spectator
     def initialize(example, elapsed, @expectations)
       super(example, elapsed)
     end
+
+    # Calls the `success` method on `interface` and passes `self`.
+    def call(interface)
+      interface.success(self)
+    end
   end
 end