util: add LayerFn which implements Layer for a closure

[seanmonstar]

Many layers are very simple, and it'd be useful to have a way to create one from a closure.

Example implementation

#[derive(Clone, Copy, Debug)]
pub struct LayerFn<F>(F);

impl<F, S, Out> Layer<S> for LayerFn<F>
where
    F: Fn(S) -> Out,
{
    type Service = Out;

    fn layer(&self, inner: S) -> Self::Service {
        (self.0)(inner)
    }
}

Need to decide on an exposed name:

• tower::layer_fn()
• tower::layer::from_fn() (similar to std::iter::from_fn)
• tower::layer::mk() (take from finagle, less clear)

[carllerche]

layer_fn to mirror service_fn 👍

[seanmonstar]

I'm kinda partial to following std::iter::from_fn...

[carllerche]

Should service_fn be renamed then?