Pluggable routing

[the10thWiz]

Implements pluggable routing, based on #2790. There are still a number of open questions, and this isn't a final implementation.

TODO:

• Trait finalization
  • Sync vs Async
  • Implicit ranking
• AsAny and DynClone
• Implement some kind of Accepts property, if possible
• Symmetry
• Open up to external implementation
• Update code generation to use new API

Async trait

I think it's worth restricting this to be a synchronous trait. It simplifies the API, and I don't think making the trait async would enable many new options.

Implicit ranking

We can adjust the collides method, to output the following enum:

pub enum CollisionType {
   /// This property ensures that there is no request that matches both routes
   NoCollision,
   /// This property allows two routes to match the same request, but
   /// Rocket should always try this route first.
   SelfFirst,
   /// This property allows two routes to match the same request, but
   /// Rocket should always try the other route first.
   OtherFirst,
   /// This property allows two routes to match the same request
   Collision,
}

AsAny and DynClone

These are blanket implemented traits that solve some minor problems. AsAny is a polyfill for trait upcasting, enabling a trait object of type dyn UniqueProperty to be upcast to dyn Any. DynClone implements Clone on Box<dyn UniqueProperty>. The AsAny polyfill can be removed with some unsafe code (basically a copy-paste of stdlib's downcast_ref). The DynClone can be removed if we use Arc<dyn UniqueProperty>, since it handles clone for us.

Symmetry

Currently, we place the burden on the implementor to ensure that the property is symmetrical (i.e., iff A collides with B, then B collides with A). Since this is only checked once, we could check both sides, and warn or error if they disagree.