Restructure stub modifiers

src/spectator/mocks/null_stub.cr

@@ -7,9 +7,5 @@ module Spectator
     # Invokes the stubbed implementation.
     def call(_call : MethodCall) : Nil
     end
-
-    def and_return(value)
-      ValueStub.new(method, value, constraint)
-    end
   end
 end

src/spectator/mocks/stub.cr

@@ -1,10 +1,13 @@
 require "./abstract_arguments"
 require "./arguments"
 require "./method_call"
+require "./stub_modifiers"
 
 module Spectator
   # Untyped response to a method call (message).
   abstract class Stub
+    include StubModifiers
+
     # Name of the method this stub is for.
     getter method : Symbol
 

src/spectator/mocks/stub_modifiers.cr

@@ -0,0 +1,5 @@
+module Spectator
+  # Mixin intended for `Stub` to return new, modified stubs.
+  module StubModifiers
+  end
+end

src/spectator/mocks/value_stub.cr

@@ -1,5 +1,6 @@
 require "../location"
 require "./arguments"
+require "./stub_modifiers"
 require "./typed_stub"
 
 module Spectator
@@ -15,4 +16,11 @@ module Spectator
       super(method, constraint, location)
     end
   end
+
+  module StubModifiers
+    # Returns a new stub that returns a static value.
+    def and_return(value)
+      ValueStub.new(method, value, constraint, location)
+    end
+  end
 end