Support nested partitions and partitions directly targeting services (#122)

* Do the nested partition thing

* Tidy up touched code

* Add nested partition test project, not fully functional

* Clean up variable names, move path_metadata mutation strictly into snapshot_reconciler

* Remove path_metadata, snapshotting is now pure

* Factor out snapshot metadata storage to fix a missing case

* Pull instance_name out of per_path_metadata, closer to what we need

* Refactor to make metadata make more sense, part one

* All appears to be well

* Cull 'metadata_per_path' in favor of 'instances_per_path'

* Remove SnapshotContext

* InstanceMetadata -> PublicInstanceMetadata in web module

* Build in snapshot testing system for testing... snapshots?

* Remove pretty_assertions to see if it fixes a snapshot comparison bug

* Reintroduce pretty assertions, it's not the cause of inequality

* Fix snapshot tests with custom relative path serializer

test-projects/single_partition_game/default.project.json

@@ -0,0 +1,21 @@
+{
+  "name": "single-sync-point",
+  "tree": {
+    "$className": "DataModel",
+    "ReplicatedStorage": {
+      "$className": "ReplicatedStorage",
+      "Foo": {
+        "$path": "lib"
+      }
+    },
+    "HttpService": {
+      "$className": "HttpService",
+      "$properties": {
+        "HttpEnabled": {
+          "Type": "Bool",
+          "Value": true
+        }
+      }
+    }
+  }
+}