Implement Syncback to support converting Roblox files to a Rojo project (#937)

This is a very large commit.
Consider checking the linked PR for more information.

src/snapshot/tree.rs

@@ -73,6 +73,13 @@ impl RojoTree {
         self.inner.root_ref()
     }
 
+    /// Returns the root Instance of this tree.
+    #[inline]
+    pub fn root(&self) -> InstanceWithMeta<'_> {
+        self.get_instance(self.get_root_id())
+            .expect("RojoTrees should have a root")
+    }
+
     pub fn get_instance(&self, id: Ref) -> Option<InstanceWithMeta<'_>> {
         if let Some(instance) = self.inner.get_by_ref(id) {
             let metadata = self.metadata_map.get(&id).unwrap();
@@ -322,6 +329,10 @@ impl<'a> InstanceWithMeta<'a> {
     pub fn metadata(&self) -> &'a InstanceMetadata {
         self.metadata
     }
+
+    pub fn inner(&self) -> &Instance {
+        self.instance
+    }
 }
 
 /// RojoTree's equivalent of `&'a mut Instance`.
@@ -371,6 +382,14 @@ impl InstanceWithMetaMut<'_> {
     pub fn metadata(&self) -> &InstanceMetadata {
         self.metadata
     }
+
+    pub fn inner(&self) -> &Instance {
+        self.instance
+    }
+
+    pub fn inner_mut(&mut self) -> &mut Instance {
+        self.instance
+    }
 }
 
 #[cfg(test)]