New snapshot tests (#134)

* Changes project-related structures to use `BTreeMap` instead of `HashMap` for children to aid determiniusm
* Changes imfs-related structures to have total ordering and use `BTreeSet` instead of `HashSet`
* Upgrades dependencies to `bx_dom_weak`1.2.0 and rbx_xml 0.5.0 to aid in more determinism stuff
* Re-exposes the `RbxSession`'s root project via `root_project()`
* Implements `Default` for a couple things
* Tweaks visualization code to support visualizing trees not attached to an `RbxSession`
* Adds an ID-invariant comparison method for `rbx_tree` relying on previous determinism changes
* Adds a (disabled) test to start finding issues in the reconciler with regards to communicativity of snapshot application
* Adds a snapshot testing system that operates on `RbxTree` and associated metadata, which are committed in this change

server/tests/read_projects.rs

@@ -1,7 +1,7 @@
 #[macro_use] extern crate lazy_static;
 
 use std::{
-    collections::HashMap,
+    collections::{HashMap, BTreeMap},
     path::{Path, PathBuf},
 };
 
@@ -53,7 +53,7 @@ fn single_partition_game() {
             ..Default::default()
         };
 
-        let mut replicated_storage_children = HashMap::new();
+        let mut replicated_storage_children = BTreeMap::new();
         replicated_storage_children.insert("Foo".to_string(), foo);
 
         let replicated_storage = ProjectNode {
@@ -73,7 +73,7 @@ fn single_partition_game() {
             ..Default::default()
         };
 
-        let mut root_children = HashMap::new();
+        let mut root_children = BTreeMap::new();
         root_children.insert("ReplicatedStorage".to_string(), replicated_storage);
         root_children.insert("HttpService".to_string(), http_service);