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/nested_partitions/expected-snapshot.json

@@ -0,0 +1,82 @@
+{
+  "name": "nested-partitions",
+  "class_name": "Folder",
+  "properties": {},
+  "children": [
+    {
+      "name": "inner",
+      "class_name": "Folder",
+      "properties": {},
+      "children": [
+        {
+          "name": "hello",
+          "class_name": "ModuleScript",
+          "properties": {
+            "Source": {
+              "Type": "String",
+              "Value": "-- inner/hello.lua"
+            }
+          },
+          "children": [],
+          "metadata": {
+            "ignore_unknown_instances": false,
+            "source_path": "inner/hello.lua",
+            "project_definition": null
+          }
+        }
+      ],
+      "metadata": {
+        "ignore_unknown_instances": false,
+        "source_path": "inner",
+        "project_definition": [
+          "inner",
+          {
+            "class_name": null,
+            "children": {},
+            "properties": {},
+            "ignore_unknown_instances": null,
+            "path": "inner"
+          }
+        ]
+      }
+    },
+    {
+      "name": "world",
+      "class_name": "ModuleScript",
+      "properties": {
+        "Source": {
+          "Type": "String",
+          "Value": "-- outer/world.lua"
+        }
+      },
+      "children": [],
+      "metadata": {
+        "ignore_unknown_instances": false,
+        "source_path": "outer/world.lua",
+        "project_definition": null
+      }
+    }
+  ],
+  "metadata": {
+    "ignore_unknown_instances": false,
+    "source_path": "outer",
+    "project_definition": [
+      "nested-partitions",
+      {
+        "class_name": null,
+        "children": {
+          "inner": {
+            "class_name": null,
+            "children": {},
+            "properties": {},
+            "ignore_unknown_instances": null,
+            "path": "inner"
+          }
+        },
+        "properties": {},
+        "ignore_unknown_instances": null,
+        "path": "outer"
+      }
+    ]
+  }
+}