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/src/path_map.rs

@@ -20,6 +20,12 @@ pub struct PathMap<T> {
     nodes: HashMap<PathBuf, PathMapNode<T>>,
 }
 
+impl<T> Default for PathMap<T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl<T> PathMap<T> {
     pub fn new() -> PathMap<T> {
         PathMap {