add `id` field to route

[stackoverfloweth]

This PR adds a new id: string field to Route type. This field will be useful internally, but is also part of the public API so hopefully useful to end users as well. The id should be unique among all routes created, regardless of if/which routers get created with the route.

Currently the value of the id is created with an auto-incrementing closure function so each id is just a number. I chose to use string for the type in case we decide to pivot to something more complex in the future.

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