Ensure mocking doesn't change the memory

This can be important for serialization.
Cleanup names.

spec/spectator/mocks/mock_spec.cr

@@ -1,9 +1,7 @@
 require "../../spec_helper"
 
-class Thing2
-  def method1
-    42
-  end
+class MockedClass
+  getter method1 = 42
 
   def method2
     :original
@@ -12,6 +10,10 @@ class Thing2
   def method3
     "original"
   end
+
+  def instance_variables
+    [{{@type.instance_vars.map(&.name.symbolize).splat}}]
+  end
 end
 
 Spectator.describe Spectator::Mock do
@@ -64,24 +66,24 @@ Spectator.describe Spectator::Mock do
   end
 
   describe "#inject" do
-    # For some reason, Thing2 is not visible to `inject` below when defined here.
-    # Thing2's definition is outside of the spec to get around this.
+    # For some reason, `inject` can't find the types.
+    # Their definitions are outside of the spec as a workaround.
 
     context "with a class" do
-      Spectator::Mock.inject(Thing2, :mock_name, method1: 123) do
+      Spectator::Mock.inject(MockedClass, :mock_name, method1: 123) do
         stub def method2
           :stubbed
         end
       end
 
-      let(thing) { Thing2.new }
+      let(mock) { MockedClass.new }
 
       it "overrides responses from methods with keyword arguments" do
-        expect(thing.method1).to eq(123)
+        expect(mock.method1).to eq(123)
       end
 
       it "overrides responses from methods defined in the block" do
-        expect(thing.method2).to eq(:stubbed)
+        expect(mock.method2).to eq(:stubbed)
       end
 
       it "allows methods to be stubbed" do
@@ -90,11 +92,19 @@ Spectator.describe Spectator::Mock do
         stub3 = Spectator::ValueStub.new(:method3, "stubbed")
 
         aggregate_failures do
-          expect { thing._spectator_define_stub(stub1) }.to change { thing.method1 }.to(777)
-          expect { thing._spectator_define_stub(stub2) }.to change { thing.method2 }.to(:override)
-          expect { thing._spectator_define_stub(stub3) }.to change { thing.method3 }.from("original").to("stubbed")
+          expect { mock._spectator_define_stub(stub1) }.to change { mock.method1 }.to(777)
+          expect { mock._spectator_define_stub(stub2) }.to change { mock.method2 }.to(:override)
+          expect { mock._spectator_define_stub(stub3) }.to change { mock.method3 }.from("original").to("stubbed")
         end
       end
+
+      it "doesn't change the size of an instance" do
+        expect(instance_sizeof(MockedClass)).to eq(8) # sizeof(Int32) + sizeof(TypeID)
+      end
+
+      it "doesn't affect instance variables" do
+        expect(mock.instance_variables).to eq([:method1])
+      end
     end
   end
 end