[rust] Add Queue (server-side playlist) implementation

- Add Queue struct
- Add API for adding tracks to the queue
- Refactors and collection navigation:
  - Switch to only symphonia backends (can reverse this)
  - Add Factori for tests
  - Rename playlist to queue
  - Refactor Collection into struct so that we can add methods
  - Refactor CollectionBuilder into it's own module
  - Define 'all_tracks()' and 'get_tracks_under()' methods on Collection
  - Add JSON errors for e.g. 404
  - Add once_cell and refactor build_test_app to fix Sled locking problems

**TODO:**

- Complete 'data' API:
- Build out category, artist, album, track GET endpoints
  - /categories
  - /categories/:category
  - /categories/:category/artists/:artist etc

Should we use IDs? hash of the path?

- RwLock instead of mutex (should be safe to read)

Makefile

@@ -1,5 +1,7 @@
 
-.PHONY: frontend go rs clean lint
+.PHONY: all frontend go rs clean lint
+
+all: go rs
 
 go:
 	cd go && make pickup

pickup.code-workspace

@@ -8,8 +8,10 @@
 			"name": "frontend"
 		},
 		{
-			"path": "pickup-rust"
+			"path": "rs",
+			"name": "rust"
 		},
 	],
-	"settings": {}
-}
+	"settings": {
+	}
+}