Support Option and Result in typed paths

[davidpdrsn]

Fixes #994

Makes this work:

#[derive(TypedPath, Deserialize)]
#[typed_path("/users/:id")]
struct UsersShow {
    id: String,
}

// this previously wouldn't compile when used with `Router::typed_*`
async fn option_handler(_: Option<UsersShow>) {}

async fn result_handler(_: Result<UsersShow, PathRejection>) {}