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_model/default.project.json

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

test-projects/single_partition_model/expected-snapshot.json

@@ -0,0 +1,53 @@
+{
+  "name": "test-model",
+  "class_name": "Folder",
+  "properties": {},
+  "children": [
+    {
+      "name": "main",
+      "class_name": "Script",
+      "properties": {
+        "Source": {
+          "Type": "String",
+          "Value": "local other = require(script.Parent.other)\n\nprint(other)"
+        }
+      },
+      "children": [],
+      "metadata": {
+        "ignore_unknown_instances": false,
+        "source_path": "src/main.server.lua",
+        "project_definition": null
+      }
+    },
+    {
+      "name": "other",
+      "class_name": "ModuleScript",
+      "properties": {
+        "Source": {
+          "Type": "String",
+          "Value": "return \"Hello, world!\""
+        }
+      },
+      "children": [],
+      "metadata": {
+        "ignore_unknown_instances": false,
+        "source_path": "src/other.lua",
+        "project_definition": null
+      }
+    }
+  ],
+  "metadata": {
+    "ignore_unknown_instances": false,
+    "source_path": "src",
+    "project_definition": [
+      "test-model",
+      {
+        "class_name": null,
+        "children": {},
+        "properties": {},
+        "ignore_unknown_instances": null,
+        "path": "src"
+      }
+    ]
+  }
+}

test-projects/single_partition_model/src/main.server.lua

@@ -0,0 +1,3 @@
+local other = require(script.Parent.other)
+
+print(other)

test-projects/single_partition_model/src/other.lua

@@ -0,0 +1 @@
+return "Hello, world!"