Validators should be able to work with references without deref() or cloning

[jasongrlicky]

Today:

let fill = FillStyle::Solid(Rgba::new(0.75, 0.5, 0.25, 1.0));
let mut d = CanvasMock::new();

// Set Expectations
d.expect_set_fill()
  .called_once()
  .with(deref(fill.clone()));

// Test
d.set_fill(&fill);

What would be awesome:

let fill = FillStyle::Solid(Rgba::new(0.75, 0.5, 0.25, 1.0));
let mut d = CanvasMock::new();

// Set Expectations
d.expect_set_fill()
  .called_once()
  .with(&fill);

// Test
d.set_fill(&fill);

I believe we can achieve this with lifetimes?