Add a single route at /race

Add a single route at /race

That returns the complete race state, which, for now, is only the name
of the race.

New rust features seen for the first time as of this commit:

  * std::sync::Arc for atomically-reference-counted pointers (with
    thread-safe reference counting as you would expect from the name).

  * "crate::" in 'use' paths to reference the root of the current project

  * impl: For creating functions associated with a struct

  * Destructuring in a function parameter: State(state): State<Arc<AppState>>

  * clone() to carefully give the RaceResponse a string it
    owns. (Note: The rust compiler wouldn't even accept the code
    without this clone(): "cannot move out of an `Arc`)