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/composing_models/default.project.json

@@ -0,0 +1,6 @@
+{
+  "name": "composing-models",
+  "tree": {
+    "$path": "src"
+  }
+}

test-projects/composing_models/src/Remotes.model.json

@@ -0,0 +1,14 @@
+{
+  "Name": "All my Remote Events",
+  "ClassName": "Folder",
+  "Children": [
+    {
+      "Name": "SendMoney",
+      "ClassName": "RemoteEvent"
+    },
+    {
+      "Name": "SendItems",
+      "ClassName": "RemoteEvent"
+    }
+  ]
+}

test-projects/composing_models/src/XML.rbxmx

@@ -0,0 +1,14 @@
+<roblox xmlns:xmime="http://www.w3.org/2005/05/xmlmime" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="http://www.roblox.com/roblox.xsd" version="4">
+	<Meta name="ExplicitAutoJoints">true</Meta>
+	<External>null</External>
+	<External>nil</External>
+	<Item class="Script" referent="RBX634A9A9988354E4B9D971B2A4DEBD26E">
+		<Properties>
+			<bool name="Disabled">false</bool>
+			<string name="Name">Lone Script</string>
+			<string name="ScriptGuid">{C62CD9FB-FF28-4FD9-9712-AD28A1E92C84}</string>
+			<string name="Source"><![CDATA[print("Hello world!")
+]]></string>
+		</Properties>
+	</Item>
+</roblox>