Add intermediate TypedStub

src/spectator/mocks/typed_stub.cr

@@ -0,0 +1,12 @@
+require "./stub"
+
+module Spectator
+  # Abstract type of stub that identifies the type of value produced by a stub.
+  #
+  # *T* is the type produced by the stub.
+  # How the stub produces this value is up to subclasses.
+  abstract class TypedStub(T) < Stub
+    # Return value.
+    abstract def value : T
+  end
+end

src/spectator/mocks/value_stub.cr

@@ -1,9 +1,9 @@
 require "./arguments"
-require "./stub"
+require "./typed_stub"
 
 module Spectator
   # Stub that responds with a static value.
-  class ValueStub(T) < Stub
+  class ValueStub(T) < TypedStub(T)
     # Return value.
     getter value : T