Issue Description:

Today, Differentiable derived conformances will treat any `let` stored property as if it had @noDerivative, because the synthesized move(along🙂 needs to call to move(along🙂 on each stored property. However, class-bound properties' move(along🙂 can be called even if they are let properties.

The following code should work.

struct Foo: Differentiable {
    let x: DifferentiableClassType
}

The property inclusion rule in the Differentiable derived conformances should be corrected to only reject non-class-bound let properties.

(Working on an implementation as I'm typing this.)