Thread InstanceSnapshotContext through all SnapshotMiddleware

src/snapshot_middleware/context.rs

@@ -1,7 +1,16 @@
+#[derive(Debug)]
 pub struct InstanceSnapshotContext {
     /// Empty struct that will be used later to fill out required Lua state for
     /// user plugins.
     pub plugin_context: Option<()>,
 }
 
+impl Default for InstanceSnapshotContext {
+    fn default() -> Self {
+        Self {
+            plugin_context: None,
+        }
+    }
+}
+
 pub struct ImfsSnapshotContext;