Rewrite examples to show mocking an API

[nathanl]

When I first looked at the Mox documentation, I found this example confusing:

test "invokes add and mult" do
  MyApp.CalcMock
  |> expect(:add, fn x, y -> x + y end)
  |> expect(:mult, fn x, y -> x * y end)

  assert MyApp.CalcMock.add(2, 3) == 5
  assert MyApp.CalcMock.mult(2, 3) == 6
end

This demonstrates that Mox works, but it doesn't show using a mock to test other code. This PR changes the calculator examples to instead mock an API interface and test the code which calls it.

[josevalim]

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