Pretify stub and call output

Fix some tests.

src/spectator/matchers/receive_matcher.cr

@@ -71,13 +71,16 @@ module Spectator::Matchers
     private def values(actual : Expression(T)) forall T
       {
         expected: @stub.to_s,
-        actual:   actual.value._spectator_calls.inspect,
+        actual:   actual.value._spectator_calls.join("\n"),
       }
     end
 
     # Additional information about the match failure when negated.
     private def negated_values(actual : Expression(T)) forall T
-      values(actual)
+      {
+        expected: "Not #{@stub}",
+        actual:   actual.value._spectator_calls.join("\n"),
+      }
     end
   end
 end

src/spectator/mocks/stub.cr

@@ -29,5 +29,10 @@ module Spectator
 
       constraint === call.arguments
     end
+
+    # String representation of the stub, formatted as a method call.
+    def to_s(io : IO) : Nil
+      io << "#" << method << (constraint || "(any args)")
+    end
   end
 end